Bosses should limit themselves to 8 hours in the office, except during times of crisis (and, even in a startup, to be constantly in crisis mode is bad management). If the work calls for more, do it from home. Why? Because if the CEO works 12 hours per day, the VPs are going to feel a need to work 13, the middle managers work 14, and the grunts have to work 15. Now you have everyone working an inefficient long day, and less is getting done (and less reliably) than if people were working sane hours.
You should lead by being available when people need you (which is not the same thing as being a doormat; your willingness to drop other things or sacrifice must be appropriate to the level of the need) but putting in mindless face time just sets a bad example. Low-level investment bankers have to suffer long face time, but CEOs? If your company is going to be a sweatshop where people feel like they need to work 15 hour days or will lose face/status, then what's the point of building it?
Startups don't change our biology or remove the fact that virtually no one can work efficiently beyond about 55 hours per week for more than a couple weeks. (That number includes commuting, housework, errands, and assorted nonsense.)