Speaking as a relatively early (#5 or so) employee in a startup, I second this. I work 40 hours a week remotely, but if I'm on a roll or I head to the office, it's not uncommon for me to put in overtime. I worry a bit that I'm underperforming because some of the other guys put in the kind of time the OP describes, but I don't think it's ever been expected of me, as long as we're getting stuff done and hitting our targets. The founders at my place may be unusually cool, however, so YMMV.