How often should I exercise?

I am over weight and am trying to lose weight at the same time as strengthening my knees (been having problems with them and now I'm having physio on them to help). I do football once a week for about an hour, and pe lessons at school twice a week for 50 minutes each. I have just bought an exercise bike and I was wondering how long I should go on it and how often? I also have a treadmill and gazelle thing, should I use those as well (My knees hurt when I run...:S) for a more even work out? Thanks