We spent three nights of Tobacco Caye several years ago. By the time we left, I was ready to swim back to the mainland. Two nights would have been plenty for us. But, we're people who like different places to eat/drink every night. We like to snorkel, but, don't spend huge amounts of time doing it. We enjoyed our stay, but, I wouldn't want to stay five nights.
I'd spend more time in San Ignacio, so much to see and do there.
Tobacco is one of my favorite places, but it is a VERY small island. If you have any fantasies about living on a small island...this may cure you of those. I usually only stop in there for a night or two on my own boat so I can leave when I choose.
If you like to snorkel, dive, fish, relax, and hang out with the locals at the bar then its a great place, but if you want a lot of different activities maybe not.Edited: 7:07 pm, August 06, 2015
Agreed with the other responders. I spent two nights there, and that was plenty. It was beautiful, and the people were friendly, all 22 residents of the island if I remember correctly. But after that, we were ready to head on to the next town on our itinerary.