This sounds about right. Though it all depends, like others have said. If it goes too long she thinks she's doing something wrong and gets worried >.> (obviously not the case)
To be honest I've never timed the act of penetration since we spend a good amount of time on foreplay. Sessions generally last 30 mins - 1 hour. Fiancee of 3 years, on the normal we'd do this 2-3 times a week, if our schedules aren't conflicting I'd want to be doing it nightly >_<.