Metformin

So I started taking Metformin a couple of weeks ago. So far, it's been absolutely wonderful! Reading other threads on here had me kind of worried about it, but I've had nothing but positive results so far. One quick question, though. This maybe stupid, but I've looked everywhere and can't find an answer. How long are you supposed to take Metformin? Is it a life long drug? What about during pregnancy? Is it still ok then?