Like Liolel, my prep time varies greatly depending on what point in the adventure we're at, and also what point in the campaign. At this point, two years into the campaign and in the middle of an adventure that we've been on for a while (with breaks for other stuff), prep time is minimal, maybe a few hours one evening. When moving into a new adventure, I probably average a couple of evenings of prep these days. Earlier in the campaign, it seemed like I was always crunching.
We play about once every 3-4 weeks in my regular (homebrew) D&D campaign, usually with approximately 3 hour weeknight sessions but with the occasional 5-6 hour weekender. Currently I'm also running a short Dark Sun story arc, playing roughly monthly. That takes a bit more prep because it's converted rules, new characters and everything.