As I've increased my frequency of flying, the amount of time I allot myself has decreased quite a bit. Depending on the time of day, the airport, the time of year, it ranges 30-60 minutes.
That said, I sleep like a baby on flights most of the time as long as my general boarding was low stress. Since I get so wound up about general lateness, it serves my desire to snooze through a good portion of my flight to get there a shade earlier than absolutely necessary. I'd rather wait a bit and sleep than get there with a few minutes to spare and be awake the entire flight. Most flights I'm out long before we reach cruising altitude, and sometimes before we even take off.