I varies WIDELY. My first daughter trained at 19 months and when she started training (we went straight to underwear and waited for her to start peeing to move her to the potty, so we know for sure), she was peeing every 20 minutes; but after a couple of days of training, she could go an hour. Our second daughter started training at 17 months and in those same first days was going more than an hour and a half before she started to pee. I think a lot of people who train using the interval method (making a point to put her on the potty every x number of minutes), usually start with 30 minute intervals and modify from there... if she isn't making it, you can make it 20 minutes, if she seems fine with 30, you can increase incrementally.
at 4:04 PM on Mar. 21, 2010