What do people consider an average work week? I do 37.5hrs religiously and have never had a problem. A good fraction of developers at head office do 30-32hrs (4 days/wk).
When I feel like I'm working much less than 40, I check my productivity. If my productivity feels good I stop worrying; if not then I try to work more. (I've been in the game long enough to have a pretty good sense of this.)
When I feel like I'm working much more than 40, I worry about my time-management and try to correct it.
I would find it completely reasonable for someone else to do the same reckoning with 30 instead of 40 hours, as long as they get shit done.
On the occasions when I've found myself stuck in crazy ruts, like six months at 80 hours per week, in hindsight it's always been because I followed some wrong lead -- a workaholic boss, an arbitrary deadline -- that I didn't actually have to follow.