When you're starting out, it can be difficult to know how long to keep your baby on the potty if she's not going. It all comes down to trial and error, and your own judgment. The quicker potty trips are, the better. If your baby is not going right away, even after being cued, you should probably take the her off and try again later. However, there are those times when baby is stressed out and needs to go, and simply needs a little time. If you're pretty sure she needs to go, keep holding her there unless she's really upset. I keep my babies on the potty if they're fussing a little during those times when I think the fact they need to go is what's making them fussy. In the early days we used our baby's feet as a timer. When they started turning purple, we'd take her off. Usually that was after about two minutes.
Try to make each potty experience a positive one. The sink is so good in those first few months because of a magical baby entertainer: the mirror. Babies love to watch themselves, the little narcissists. This can help your baby associate going potty with relaxed happy feelings, so the by the time they're getting too heavy to hold on the sink, they'll be comfortable with going potty in general. This will make your trips quicker, and you won't have to question whether you're keeping them there too long.