how long should a restore from backup take?

daughter's iphone 4 8gb had 2.5 gb of other so we decided to restore from backup, it did the restore part we could see but now the screen is blank and the phone is not showing in itunes. it's been about 30 minutes. i don't want to touch it because i know that could mess it up, but at what point do we realize we have to try something else? thanks.