No, she isnt done. She has some more fight in her. All the greats lasted a pretty long time.... Graf was winning slams at 30 and Nav was getting to Fs in her late 30s. Serena is more physically gifted than either of the other two ladies, so I cant see why she cant continue for a while longer. Whether she wants to or not, thats a whole different story.
Graf won her last slam just before her 30th birthday, and it was a gift from a self sabatoging Hingis. Her first major title in 2 and a half years, obviously now well past her prime. Then soon after turning 30 she was badly overpowered by Davenport in the Wimbledon final and retired.
No player since 1990 has won a slam in their 30s. The game was nowhere near as physical back then as it is today.
I could see Serena winning anywhere from 0-3 more slams. However the more I think about it the most likely answer is probably 0.
Serena is not more physically gifted than Graf and Navratilova. She is a more gifted ball striker than either probably, and more gifted server, but in terms of pure athleticsm those two are the best in history. Serena isnt even the most physically gifted of her own era, that would be Venus. Even if you disagree she is about 30% as dedicated to fitness and staying in shape as Graf and Navratilova, has had more injuries than Graf and many more than Navratilova, and as a result at this stage of her career is nowhere near as athletic as they were. Watching her matches since her return to injury her movement around court is now average at best, and she looks clumsy at times. She also tires easily, and doesnt have an abundance of energy in the tank if needed. Even when she wins like in Canada it has nothing to do with athleticsm anymore. Her groundstrokes are still solid and effective, though nowhere near as overpowering as once. She mostly wins on her GOAT serve, sheer will to fight and win, her experience and improved point construction, and excellent return of serve. When someone is either overpowering her, getting everything back, or mixing patience with hitting early and accurately enough to move her, she becomes almost useless now as she did in her mostly one sided losses to Bartoli, Stosur, and Makarova since her return.