Even if long hours are called for, I assume it is flex hours?
During the mega crunch times at the start-up I was at, I'd come home and eat dinner with the family and play with the kid. After they went to bed, I'd sometimes work from home, or even go back in to the office if necessary. Question is how much sleep do you need? I do ok with 6.5 to 7.5.
If you're really really concerned about it, after they've made you the offer you could tell them that you are super excited and interested, but that you have this one concern and want to know what their expectations are, and what common understanding you might be able to work out.