Since I did my studying purely during evenings and weekends, this is really just an educated guess, but two or three months per module sounds reasonable if you really do go for it.
The main reason for posting, though, is to make two points. First of all, will you really be able to study full-time? I'm not just talking about outside influences like your wife insisting on you spending time with her occassionally, I'm also talking about whether you'll be able to find the motivation to work at that rate. Bristol's second module is so boring that I can't find the words to explain how hard it is to keep going - and I'm saying that as someone who only needed to spend a few hours at a time on it, not days on end.
The second point is that you should make sure your school are aware of your time-scales. I don't know about Oxford, but the brush-up course at Bristol gets booked a few months in advance, so you'll need to book it as soon as you start studying, if not before. Have a chat with them, let them know what you're planning, and see what they advise.
Good luck!
FFF
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