First of all - you might want to consider getting on Xbox LIVE Indie Games (XBLIG) first, and trying to get on Xbox LIVE Arcade later. The latter requires a proper publishing agreement with Microsoft, the former does not.
Giving you a time estimate is very difficult, as has been mentioned. Especially without knowing your level of experience. My last finished game was a technically-fancy platformer, released on XBLIG, with a team of one, and took 4 months to develop. But I started that with years of experience - and I had to make huge cuts in order to fit inside that time limit.
If you have little or no experience, I would suggest starting small. And when I say "you" - this also applies to your team as a whole. Start with making a very small game (tetris, snake, that sort of thing) or two, from start to finish - including distribution of the finished product. This will give you some critical experience and give you a better understanding of what goes into making a game and how long things take.
Finally - rather than estimate - I would recommend you set yourself a time limit and try to schedule inside of that. For an inexperienced team making a platformer, I would say 6 to 9 months is reasonable, depending on how complex you will make it. (This assumes, of course, that you take my advice and try a small game first. For a small game set a limit of, say, 1 month.)