Maybe ask her to give you a call at a certain time later on in the evening to check in so she can tell you if she knows how long she will be out. For the first few dates, I think it is natural to want to know the specifics of what you can expect. I think as time goes on, people can get use to more general plans.
While she is out, do something fun for yourself. Don't do anything where you may just watch the clock or obsess on what is happening.
If you didn't want a curfew, then you can unserstand why she doesn't want one. Maybe now she can appreciate what she was asking for when she wanted one from you from before. But if neither one of you really wants a curfew, then get rid of the idea now while you can.