On the 22nd of March, me and my girlfriend will be traveling from Antibes to Florence by train. Since the journey is a long one, we plan to make a stop at Portofino, and then take an evening train back. We will be starting in Antibes at 7:21am and arrive in Santa Margherita Ligure-Portofino at 12:04pm.

The problem with our original plan is that, since we have booked the tickets that departs Portofino at 20:16, it will arrive in Florence at 23:50, and we are running into issues with our current hotel booking, which tells us that it is hard to find accommodations in Florence that checks us in after 21:30. I have the option the refund the trip from Portofino to Florence, and book an earlier train, here are my options that arrive before 21:30:
(portofino departure time--> arrival time in florence)
15:15-->18:01
16:02-->19:33
17:15-->20:33

How much time should I spend in Portofino? If I were to choose the departure at 17:15, could anyone provide me with an itinerary for around 4hrs30mins to see the best of Portofino?

Welcome to Fodors. We modified your title to clarify your topic, and also tagged your thread for Italy.

Two hours is enough for over the top expensive Portofino. Spend more time in lovely Santa Margherita Ligure.

Portofino is a tiny little village, extremely pretty, full of expensive but yummy shops and expensive restaurants. If you are looking for designer shopping you could spend a fair bit of time here plus lunch. We had lunch at Stella (recommended to us by a local), it is right on the port so pretty location and good people watching. Yes, it was expensive but the service was flawless and the food was really good. Or you could walk up the hill to the Splendido and have lunch with a view-this will be expensive too but gorgeous.
‘If shopping is not your thing then I suggest lunch at Stella and a walk around before heading back to SML and on to Florence. This will take less than four hours.
There is a bus that runs between SML and Portofino.

SML to Portofino is also walkable.

Portofino is one of those places that sounds better than it is. It's a pretty enough small village in a pretty enough cove, but as mentioned, if you're not eating or shopping it's a half hour--tops. People get this romantic notion of something out of a fairy tail, but it isn't. It's pretty but that's it. The bus ride from SML takes about 10 to 15 minutes, parking is difficult if you're driving. I'm not sure that I'd take the detour you're proposing just to see Portofino.

Portofino is a really small place that you can see in about 1-2 hours. Of course it is very beautiful there and it is also worth staying there overnight and enjoying the ambience for longer! Unfortunately, Portofino is also very very expensive!!!

Personally, I love such places and the sea and would stay there as long as possible

Don't forget to allow time to store and retrieve your luggage.

If the weather's nice, I would walk the path between SML and Portofino via Piaggi at least one way.