Depends on what the topic is and how long ago it was last discussed.
If it's one of the "hot button" topics (including but not limited to WoW, Half Life, anime, JRPGs, and best/worst story/dialogue/character/muffin EVAR), no more than once every three weeks or so, really. If you're really curious to know what people think before then, suck it up and read the frackin' thread. No sense in restarting old, annoying arguments.
In fact, three weeks might really be the lower limit. There is no upper limit; I've never been on a forum that explicitly allowed thread necromancy like this one.