You'll probably know when she's had enough when she has out of cage time - but yes 15 to 20 minutes is a good rule of thumb (pardon the mention of thumb
).
If you have a particular thing for getting her out (eg a tube or a ball or I use a fleece cube ours walks into to get him out of the cage) and you leave the transport vehicle in the play area, they will sometimes climb back into it to tell you they've had enough and want to go back.
If you can set up an out of cage area for some taming (the bathtub is good) with a couple of tubes and a hide and something like a flying saucer wheel, they can enjoy running round in it and you can do some hand taming as well.
I used to use a hamster ball to get them out (if it will fit in). Put a treat in it, they climb in. Pop the lid on and carry the ball to the out of cage area and set it down and take the lid off. If you can prop it so she can climb back in it again easily, without it rolling over then that's the kind of thing they hop back into when they've had enough,
I now use a snooze cube. Which is a square fleece cube - it's cosy and they like climbing into it - so I use that as the transport (with my hand over the entrance).
If it's any consolation our new Syrian just nipped me twice tonight! Had my hand flat on the floor of the playpen for him to walk over and twice he just nipped it! Testing it out probably to see if it was edible. He will accept a stroke on the back once or twice, but not being picked up or hands just yet. It takes time. Once they get used to a stroke now and then and get used to your hands not being food.
Maybe try having the palm of your hand flat up (ie flat with palm up) and when she walks onto it, just raise your hand slightly about 1cm or so but let her walk off. If you do that a few times, then do it slightly higher each time, you should be able to hold her after that and let her walk from hand to hand. But I wouldn't do it in her cage/bin. That's her territory. Another area for taming/out of cage time is better.