I had this, might be resolved now though. I aborted the first attempt, but found some stuff on the iPad, so it may have been working, just looked like it wasn't.
So I tried again, same behaviour. Then I read the first reply to your question, 'are you a developer', and closed XCode. Within several seconds a time estimate appeared. Wandering from 20 to 40 minutes, with a progress bar.
So I figure closing XCode might have resolved it.
So, are you a developer? XCode running?