It's not based on age.
You can start anytime if you are home all day with her and can stick to a rigid schedule (which means few or no trips out of the house). You'll need to take her to the potty every 30 mintues and keep her there for a minute or two. Eventually, she'll accidently go. When that happens, you heap her with praise. Eventually, she'll grasp the concept and start letting you know when she has to go.
You can wait until she is ready. Most girls are ready sometime after 24 months. You have to evaluate your daughter to see if she is capable of holding her pee and figuring out if she has to go. BabyCenter.com has a good list of the signs.
No matter which you choose, how long it takes varies from child to child... it could be quick (a couple weeks) or long (over a year).
at 11:43 AM on Nov. 8, 2008