How much time should I spend in Boston

I recommend the Isabella Stewart Gardner Museum (right close to the Museum of Fine Arts), and the New England Aquarium. I imagine you’ll be exploring the Boston Common - I particularly liked the western end, with the swan boats, the make way for ducklings statue, etc. Oh, and in case you want to venture into Cambridge, you could visit the Harvard art museums.

Harvard also has the Museum of Natural History and one exhibit I would like to recommend.
I first saw this written up in Martha Stewart magazine oh, 15 years or so ago. My teen daughter and I were going to be in Boston and area and made a point to see this fascinating exhibit. I am glad to see it is permanent https://hmnh.harvard.edu/glass-flowers
It is a collection of glass plants that were used for training. Though understandable, I think we forget how difficult it must have been to learn botany when plants die so quickly. These glass pieces were made to represent the real thing, with cross sections, blooms, seeds, fruits. Beautiful works of art and labour of love over decades.

If you enjoy living history museums (love them, myself), you could do a day trip to Plimoth Plantation in Plymouth, about an hour away by train or bus. It’s one of the serious ones, where asking the reenactors about anything beyond their time period gets you nothing.

The places I thought of have already been mentioned, but to add a bit of detail: I think there's a minor discount when you see the second of the two art museums (MFA and Gardner); check the websites to be sure it doesn't matter which one you see first. Be aware that the MFA is quite large.

As a point of background interest, it's worth Googling to get the story about the major theft from the Gardner Museum.

• Acorn Street. Supposedly the most photographed street in the US. https://people.com/travel/is-this-the-most-photographed-street-in-america/
• There are several tours you can do of the Freedom Trail. Very Interesting. https://www.thefreedomtrail.org/
• Fenway Park? I'm a huge baseball fan so this was a big deal. Wasn't in town when the Red Sox were there but I did do a stadium tour.

If you have Colonial era ancestors; you can find burial sites on Find a Grave if you have First Name and Maiden or Surname with at least an approximate birth or death date. This can take you to their home town or tell where they came from before arrival in New England. I have enjoyed using Ancestry to find several lines of diverse people. It's fun to find out what they did for a living. In Boston, I had a lot of blacksmiths who made everything from canons to architectural decorations. And some clock and watch makers, and some drapers who imported and sold cloth. The Mothers often had up to 12 children so the families could make connections through marriage and could keep the businesses going. Bon Voyage!

My Daughter lives in Boston and we have visited several times. Boston in August is nice (don't go in the Winter).

Don't miss visiting the USS Constitution and its museum. The Museum of Fine Arts is great. Also, there are other nice museums.

Also, you can take the train to Salem. We spent two nights there and loved it. You can probably do a day trip and see most of what you want to see.

Paul Revere's House and Bunker Hill are interesting. Lots of great Italian food in North End.

I never miss a boat ride if I can help it, and there are harbor cruises and more in this category. August will make you want to get out on the water where it's cool and breezy!

With all the good suggestions above, I'd guess about five days should do it, more if you want to day-trip to Salem and/or Plymouth (in opposite directions from Boston).

For a day trip , take the T ( commuter rail ) north of town ,. to Rockport . It's a charming New England seacoast town , and while generally thronged with visitors , it's still well worth the time , Take a look - https://youtu.be/RDfGZctARtE

The JFK Presidential Library and Museum outside (south) of Boston. Hopefully it will reopen from its Covid closure by the time you're there.

In addition to the Gardner museum you must also allow half a day for the Peabody Essex Museum in Salem, plus more time to knockabout Salem itself -- they retain a crusty attitude towards tourists that is a nice contrast to the way you will be catered to in Boston.

And for getting back, if the weather is good there is a small ferry that is a great break from the commuter trains.

And don't forget while at Boston Common to have at least dessert (Boston Cream Pie) at the Parker House, now owned by Omni
https://www.omnihotels.com/hotels/boston-parker-house/property-details/history
where Ho Chi Minh worked in the kitchen as well as Malcolm X
https://historyofmassachusetts.org/omni-parker-house-hotel/

August is a great time of year for whale watching too. The Boston Aquarium offers tours which I have taken and found excellent. I did get seasick but my SO did not.
Even with feeling seasick I would do it again. A wonderful experience and memory.

I'll admit getting a little misty-eyed visiting the historic sites in and around Beantown. Makes me admire the sacrifices that led to America's birth.

Deborah, excuse me but I AM NOT going whale-watching again and getting sick as a dog! It is not worth it imho. I embarrassed myself and my wife and was even laughed at while exiting the boat. It was humiliating. One man said, "They should make you go back in and clean it up!" Well.

I would recommend the Boston Trolley Tour which is a hop on hop off trolley. It goes to all the major sites such as the Old North Church, Paul Revere House and Old Ironsides. You will have to pick and choose which sites to stop at as you won’t be able to do all of the ones on the tour. I would highly recommend doing the tour of Old Ironsides. Boston is a great city. Enjoy.

I've lived in and within 10 miles of Boston for 45 years. I'm still not finished with the art or history of the area. So as for how many days...I couldn't tell you. LOL. The Museums mentioned by others above are well worth the visits.
-IMO the Seaport District won't take you long...it is still pretty soulless, being so new and only now starting to install businesses to make it a community. It was more interesting before they developed it.
-Boston is a great walking city. The Freedom trail is interesting, of course, and will take you most of the day, and should be seen as it will bring you by/to most revolutionary sites. If you can grab the map that shows the original water line of the city as well, that would be interesting to have as you walk the Trail, but you might have to ask for it specifically as it is not well know the last time I heard. The TI on the Common might have it.
-If you have a particularly hot day, you might want to take the ferry and go to Georges Island or a few of the Islands or just take the ferry around the harbor. The islands are a National Park. Georges Island would be one destination with Fort Warren on it...but I'm not sure when you can get into it and if they always have interpreters there. Something to check out.
-Walk Beacon Hill, but then walk around the Back Bay as well...down the mall on Commonwealth Avenue, and also down Newbury Street. There used to be quite a few upscale Art Galleries along Newbury Street, but I'm not sure what affect Covid will have had on the street.
-Also, in the Back Bay, on Boylston Street is Trinity Church by Henry Hobson Richardson, which you might want to visit.
-Walk the Esplanade along the Charles River and see what is playing at the Hatch Shell in the evening...bring a picnic.
-The Old State House has the history of the Ancient and Honorable Artillery Company of Massachusetts if that is something that interests you.
-Walk through Quincy Market and stop into Faneuil Hall if that interests you...won't take long. Head to the waterfront and walk the Harbor Walk. Rowes Wharf Hotel used to have movies and music outside on the waterfront on some evenings in the summer. (Again, the Covid thing so I don't know.) Walk the Rose Kennedy Greenway to the North End and have dinner. Have an Italian Pastry, but walk a street back from Mike's...that's too much hype and not as good as Bovas or even the Modern.
---I could just go on and on, but I'll stop in the city for now.
-Easy day trips: Overlooked>Take the subway to Quincy Center to the Adams National Historic Park. Take the trolley to the birthplace, summer white house and burial of John Adams and John Quincy Adams and the UU Church/Quincy Temple has a beautiful interior. (Good Indian food at Sher a Punjab in Quincy Center.)
-Plymouth Plantation is wonderful, IMO, but you really need a car as I don't think the commuter train is practical for that.
-Provincetown: a long day, but you can take the fast ferry from the harbor out and back in a day, and enjoy Provincetown for the day, people, art, food, cool ocean breezes, beach, etc.
-I wouldn't put Salem on my list for a first trip. Actually it took me years to get there. LOL. Kind of interesting, but a lot of Witch hype. You can find the same age buildings in Boston to a higher degree. IMO

I'll stop now, but I could go on... Feel free to PM me if you have any questions.

Oh sounds like you had it bad Mike.... sorry.. Fortunately I never got that far... just had the quesiness. I had taken a Bonine before hand so maybe that helped. Most people were fine. Only a small number of us who weren't.

Still to see them come up right by the boat. Thrilling!

If they’re running this year, Google walking food tours of the North End (Italian section of Boston). I think the Freedom Trail goes thru the North End, but we also did a walking tour of just the North End. Really enjoyed it. Also, someone may have already mentioned, but the main library is beautiful for its architecture and is a nice place for afternoon tea (again, if they’re having it this year.)