is a day trip from Florence to Cinque de Terra worth it?

I don't think it would be worth it. Look at the time spent on train at Trenitalia, or share what the our specifically involves. I like to keep day trips to no more than 45 minutes in transit each way, but everyone has their limits.

If it were me, I'd forget about CT this time, and either spend the entire time in Florence, or consider a day trip to Fiesole, Siena, or Lucca.

You can get to Fiesole from Florence on a city bus; it's that close. There are several intersting things to see there (Roman ruins and a museum,) and great views. Siena and Lucca are great day trips as well, although both of them really want more time than that.

If you choose Siena, that's a trip to make by bus, rather than train, because the train station in Siena isn't conveniently located from the tourist's standpoint. Lucca's a train ride from Florence.

To get from Florence to Monterosso in the morning, the fastest train connection takes 2 hr. 49 min. Schedules vary by day, so you might need to leave rather early or spend more time on the train in order to arrive by noon. Most of the return trips take between 2-1/2 and 3 hours, so you'd be spending between 5 and 6 hours dealing with trains just to get to one of the five towns, and that doesn't count the time you'd spend traveling back and forth between your hotel and Santa Maria Novella station in Florence.

Definitely not worth it.

You can check the train schedule here: https://www.trenitalia.com/en.html. Your origin would be Firenze S.M. Novella. The first Cinque Terre town would be Monterosso (not Monterosso Marche).

Our own post tells you why it won’t be worth it
Very crowded
Impossible to get around
Hordes of people

that's what I thought..(re. crowded)....I was planning on going on a paid excursion with a group travelling by van I believe....which got good reviews, but then, they may have gone during a less busy time of the year....I'm thinking I'll prob just stay in Florence, seems to be plenty to do..

thank you everyone

ViaMichelin (not known for pessimism) estimates the driving time between Florence and Monterosso at 2 hr. 22 min. It's hard to imagine a van (which might need to make a comfort stop) would be faster than a car. So you're still looking at 5+ hours in transit to get to one town, plus the time it takes to get to and from the van pick-up point in Florence. (If the van picks you up, how much time will be spent driving around the city, picking up and dropping off other people?) And on a tour, the decision about when to move on from the first town to the second (and so on) wouldn't even be yours.

I was lucky enough to learn about the Cinque Terre early and see it before it was discovered by hordes of people. (This was before Rick published his first book.) It is a pretty area, but if I return, I'll allow enough time to take the boat along the coast rather than just depending on the sardine-can train. There are other very attractive Ligurian towns (not necessarily much easier to reach from Florence) that are not undiscovered but are not swamped like the Cinque Terre. It's worth learning about them, too.

If it wasn't June I might say yes .... but June is so crowded. Learned that one the hard way. Sadly, we were there on a National day... yikes!

It's a 6 hour journey round trip, when everything is said and done. June is very crowded. Only you can tell whether it's worth it or not. It depends on your priorities. You are staying only 4 nights in Florence, that is 3.5 days at best. You can easily find a lot of things to visit and do in Florence in that time. If you absolutely feel the need to take a day trip (but no more than one, since you'll need at least 2 days to see Florence's basic sights), I can think of several places in Tuscany that you can visit, before going all the way to Liguria. For example Siena, Lucca & Pisa, Pistoia, Arezzo, Cortona, just to name a few Tuscan popular cities. If you are interested in smaller towns San Gimignano or Cortona are within easy reach by bus and train respectively. You could also visit the Chianti Hills area, an idyllic wine producing area between Florence and Siena, but for that you probably need to rent a car or buy a 4 hour excursion with a private driver.
If visiting a pretty coastal town and having a beach day is an absolute must for you, then you should consider Castiglioncello. It's much closer.

We were considering this also but decided to do a day in Venice instead. We will only have 6 hours in Venice but still fun and excited to have my teenagers see it. We arrive by train from Florence at 10am and leave at 4pm will be back in Florence for dinner and sunset. Will certainly be a long day but I love Venice and although quick it’s better than nothing. Maybe that would interest you? They do have later trains so you could spend longer there.

I've decided not to do it...thank you everyone...

Although everyone advised not to do it, I did do it. I took the fast train to Cinque Terre from Florence. I hiked, saw all of the 5 towns and then took the train back. Hiking in CT was on my bucket list and I did not mind the scenic train ride. Only you can decide what is worth doing.
I decided to go on my own, I was traveling solo, as I wanted to go and see things at my own pace. The train was easy to figure out and relaxing to me.

Amy, when did you do this? what month of which year? thanks!

Following up on Amy's post....
If you were to take this day trip, I'll advise doing it as she did: independently, by train. My personal gut is that having to trail a group leader through the CT summer crowds would be NO fun at all. You'll move more nimbly on your own, and be able to spend your time there according to your own preferences.

It looks like her adventure was in August, 2018 (hope it's OK to link your thread, Amy):
https://community.ricksteves.com/travel-forum/italy/cinque-terre-for-1-day

But if put off by masses of people, you might want to consider some of the other good options suggested? Along with offering more breathing room, some of them will involve spending a lot less time on transport.

If you decide to do it, the fastest option is to take the FrecciaArgento (fast) train in the morning from the Florence Campo di Marte Station. It departs at 7:54 am and arrives at La Spezia at 9:37am. From La Spezia then you would take any of the numerous Regional trains to the village of your choice. By choosing that option, you could be at the Cinque Terre around 10 something am.
That would give you a full day to enjoy the place. I think the last train that can get you back to Florence before midnight departs from La Spezia at around 9pm.
Check train timetables here:
www.trenitalia.com
As departure station choose FIRENZE (TUTTE LE STAZIONI) which means Florence (All Stations). The arrival station is the village of your choice.

Roberto, that makes a lot of sense! thanks for that useful info......now how to turn off

We drove there from Volterra for the day. It was early September. Hiked between four of the towns, hopped the train to the last and went for a swim. We had lunch somewhere in the middle of it all. It was a long day but it was worth every moment. We still look back on that day as one of the best of our lives.

Now, I wouldn't do that sort of thing for the Amalfi Coast, for instance. The good thing about CT is it is easy to get to. Then you just park your car and train or hike. So really all the hassle is on the front end.

Also, we stopped at some random little town on the way and bought a whole focaccia and cappuccinos at a bakery with two little old ladies who only spoke Italian. When we got back in the car the focaccia's oil had made the paper bag clear. It was so salty and oily, total heaven! That day was exactly perfect.