Back in HS and Jr High (1st edition) we would meet as soon as we were out of school and play until someone had to go home for dinner, usually from 2 to 6, so the 4 hours was spot on. A lot of times we would continue after dinner though, so they usually went longer. I also started a D&D Club, but we only got to meet for 45 minutes a week (and they played evil characters, so much of that time was inter-party squabbling)...
More recently (3.x era), I had a Wednesday D&D group for a while that would meet for about 4 hours, from 6 to 10. When I ran a Friday D&D game at the big LGS, we would meet from 6:30 to "close" - usually 10 to 10:30, so that usually clocked in at 3 to 4 hours (depending on set up).
As for my current game (and other RPGs):
Sunday D&D (every other weekend): 6-8 hours, starts at 4 and ends at 10. Which really means we start at 5 and end around midnight, sometimes later.
Sunday Serenity (alternate weekends): 3 hours, starts at 5 and ends around 10 or 11. It's not really what you would call "focused".
Friday Savage Worlds: 4+ hours, starts at 6:30 and ends around 11 or whenever the GM realizes how late it is. Technically, we had agreed to stop at 10, but we have never taken that seriously.