I've never beta tested a game, but have tested lots of other software, both alpha and beta. Testing times have varied enormously from several months to only a couple of weeks. It is very difficult in programming to put a time on a release. All can seem to be going well, then a bug is reported which means scrapping huge chunks of code and starting nearly from scratch.
Although I don't code actual programs at work, I am involved with some 22,000 users constantly bugging us asking when (exactly) a new release will be coming out, and I feel like replying "When it's ready. If you stopped asking it would be even sooner as we'd have more time to spend doing what we should." However, I like my job too much.
I would say, though, that we are getting dangerously close to the year-end.