It's fine for dark beers. Depending on how much longer you have to boil it might darken the color and change the flavor of your lighter beers. Extra 15 maybe even 30 minutes would be fine. An hour is definitely too much. If you consistently overshoot your pre-boil volume, you should be able to just cut some water from your strike water and get it right. If you miss by half a gallon then cut your strike water by half a gallon (make sure to compensate on the temperature though)
Beer is proof that God loves us and wants us to be happy.
- Benjamin Franklin (and I don't care if this quote has been largely discredited/misquoted...I like it!)
Beer, if drank with moderation, softens the temper, cheers the spirit and promotes health.
- Thomas Jefferson