Through Eternity Early Morning Vatican Tour

Perfect tour for a prospective Art History major -- it separates the men from the boys. Quite a slog, in a good way.
There is a lunch break when you try to grab a table or two in the cafeteria (worst meal you'll have in Italy)
and a couple other moments when you're passing by WCs.
Those seeking extra credit can stick around for mass in St. Peter's.
I recommend doing your homework ahead of time -- read up on Titian and El Greco, know how we got to then, where we went afterwards ...

With an interest in art history, I would strongly suggest you look at the website for context travel. The guides for their art tours are typically PhD-level art historians. For example, when I took their Vatican tour, the guide was an American who had lived in Rome for many years, has a PhD in art history with a specialty in the Baroque, and teaches in university programs in Rome. I had visited the Vatican on several previous trips, but I still learned so much on their tour. Their groups are limited to six, and will go with fewer. They are a little more expensive than other walking tours, but in my experience, they are worth it.

I've taken several of their tours in Rome, and have also use them in Florence and Venice (as well as other cities). All of my experiences with them have been really good (and have been far superior to my experiences the few times I've used other walking tour companies).

I did a Through Eternity tour last year, excellent guide. There were bathroom breaks as needed, plus about 30-40 minutes for lunch. Lunch will be okay if you stay away from the pizza. Our guide ran over the schedule, no one minded.

In addition to the Vatican Museums which really are museums of the various Popes' collections and gifts, for an Art Historian the ultimate would have to be all the art "in Situ" in all of the churches in Rome (and other towns and cities in Italy) which is where the artists were commissioned for, executed (especially as most were frescos) and placed the art. For example, in Santa Maria sopra Minerva (next to the Pantheon in Rome), which I visited last week, there are many Bernini sculptures, including a fabulous happy elephant with an obelisk in the square outside, and a wonderful Risen Christ by Michelangelo. and many excellent paintings, including several by Filippino Lippi, all right where they were supposed to be. There is also the tomb of St Catherine of Siena, the patron saint of Italy, and the tomb of Fra Angelico. That place is free to enter, a few euro coins to switch on the lights for better views of the art (unless somebody beats you to it) , and spellbinding. I think we were there about 2 hours this trip.

Or going down into the 4th century church under the 12th century church of San Clemente and seeing how the art and painting has changed over 1000 plus years, and then to go down the last set of stairs and find the pagan Mithras from the 1st century under that. That's history; that's art history.

Then there are Bramante's cloisters at Santa Maria della Pace, with the frescos above each doorway. That's free, too, but the coffee and pastry upstairs on the first floor at the cafe isn't. But you do get excellent coffee, a great atmosphere, lovely relaxing music, and sitting in the nooks overlooking the courtyard of the cloisters below - an excellent view.

The Bernini sculpture of The Ecstasy of Saint Teresa is in the Cornaro Chapel, Santa Maria della Vittoria. Got to be seen to be believed.

So, perhaps, get her around to some of these churches. It involves a bit moving around the city but is well worth it. And cheap, too.

I'll second the Context Travel recommendation -- I've been on their tours in several cities and will adjust my own schedule to fit into theirs without hesitation. On their Renaissance tour in Rome, I was the only one in the group who didn't have an MFA. (I could still keep up, though. Always ready for a challenge.)