I'm currently running a 3.5e campaign based off early '80s Classic D&D modules (Rahasia, Horror on The Hill, The Lost City, etc) - these seem to be around 4-5 5 hour sessions each, which at 2 games/month suits me well. When I ran 3.5e Lost City of Barakus though it took nearly two years (February 2005-December 2006), around 40 sessions I think, and that was way too long for a single dungeon crawl. I like the old White Dwarf magazine adventures, around 5-6 pages and usually completable in a single session. I think if I were only playing once a month I'd definitely prefer one-offs completable in a session, with the occasional epic two-parter as a "season climax" type event.