I've used 2 different ones for contest, but you can count post in x forums in x time with a little editing on this mod.
Top Posters for a Specified Period.
and this one is still in BETA I think, but you can select usergroups, forums, time period and you can set "points" for long post, or threads started..
Easy Contest is pretty cool, but like I said, I think it's still in beta.. You'll just have to ask the developer..