There aren't any statistics so it's pretty much impossible to give you proper answers to your questions. For me, I received my offer (I'm an international) within a week after I submitted the questionnaire back. However, I think the quickness was because I was placed in the same "batch" of offers being distributed on that day - I've seen people who had to wait a week or a few more before they got their open day invite or offer.
I honestly can't say what the chances are. In general getting the questionnaire is a sign to show that you're at least over half-way there. So far, though, I haven't seen anyone who's gotten the questionnaire and got rejected.