I don’t think your family tree ever goes away. There are some out there from the beginning of time, it seems, and they appear to be long since abandoned. But they’re still there.
I don’t know what happens if you let your subscription expire, but I think they keep the trees regardless. That info might become their property, but am not sure; you would have to check the agreements. My tree is still there after a few months of not signing in (although I’ve never let the subscription lapse).