I think my longest playing time is maybe 10 hours a couple years ago. I'm lucky to get 4 hours of straight gaming time in now. I usually play about 2 hours straight and then take a break, then repeat. When I started playing back in 2006 and was still F2P, I never played all that much, but once I got membership, I started playing non-stop for hours on end. But now that I'm in college, I only get to play during the summer. I'd love to play for 10+ hours on end, but I have to take breaks now because I either get bored of playing or want to go watch a TV show or something like that.
