i push my machine pretty hard, so only a couple weeks. i was forced to restart last night because warcraft flipped out. my record is about a month, but by then i start seeing weird interface quirks, or permissions are screwy, or something won't run right. if logging out and in doesn't fix it (or repair permissions, depending), i restart.
started having this thing happen lately, where after a couple days i can't select anything on my desktop - i can select, apparently, but nothing looks highlighted - and any overlays for dragging files or images, like from a browser for instance, don't work right or at all. relaunching the finder usually fixes it.