Mine took about a month to settle down.
I almost thought it wasn't going to, and was resigned to having it float, when one evening I came home, and it had finally chosen to settle down to the bottom.
I've had it in my aquarium for almost 6 months now, and it's still fairly buoyant. (It's touching bottom, but the middle bend refuses to settle on the gravel.) The overall effect isn't too bad, so I don't really mind, but once my fish gets a bit larger and I have to upgrade to a bigger tank, I think I'll look for heavier wood, maybe that malasian stuff people seem to recommend both here and in other fish sites.