I generally try to get at least seven and a half hours. But quite often it ends up being six, or even five, or four. I know from experience that I can generally go for four or five days on four hours of sleep before I hit the wall, but I've never actually discovered my real hardline limit, which kind of worries me.
I'm an inertia kind of guy - I have a hell of a time getting going in the morning, but once I am, I'll just chug along at whatever speed is required all day long until I just fall over dead. Which usually means I have a hard time unwinding and stopping at the end of the day.