Hmm, yes and no.
I had maybe 2 posting contests years ago: the first one was pretty short and people would just come and post for the points. Then, as the contest ended, most of them left.
The other one was I think 3 months, with some pretty decent prizes (don't think a lot of money, but a few hundreds bucks, advertising on the forum (which had a pretty decent traffic), I was active like crazy on TAZ, ForumPromotion and a few other forums that now are no longer active.
In those 3 months I got some people who were clearly for the money (and that was OK with me), but also quite a few who loved the community itself (again, posting super-high quality content really allowed us to get noticed) and stayed for years.
So, if I had the time/money I'd consider it, but maybe make it a little longer, so that you can get at least some of them to love the community and consider staying after the contest is over.