Gosh, that could be us! We have a 2.5 year old girl who uses the have-to-potty thing to stall bedtime right as we are ready to turn the lights out. Except we aren't really doing the night training since she is still waking up with a wet diaper. Anyhow we just let her go (what's 15 minutes really?). I don't ask her if she's done and I don't go in there. Once there's no interaction she gets bored and comes out saying she's done. If the 15 minutes or whatever is a big deal in your situation, maybe you could just finish the bedtime routine 15 minutes early in anticipation of this **new** bedtime routine?
If she's asking right after you leave the room or everytime you get her back to bed I would tell her that part of the bedtime routine is to use the potty before she goes to bed and make it the last thing she does before you sing/rock/put her to bed.
If she's doing it in the middle of the night, I would just make it possible for her to go to the bathroom without your assistance. Make a safe path from her room to the bathroom, have a step stool set up by the potty and her seat set up. I know a 2.5 yr old who goes all by her self when she wakes at night and then goes back to bed when she's done. My daughter, however, is not ready for this yet. Incidentally, my daughter also did alot better using the potty by day when I stopped asking her if she needed to go all the time and just set up the bathroom so she can do it by herself.
Good luck and let me know what works. ;-)