How often do you do leg machine exercises in a week?

Hi. I would like you to share your ideas with me on this. Bearing in mind I have over-exercised by playing Football too often. My knees apparently don't have problems as such but they are worn out from over-exercising. Sometimes my knees give way when I'm walking and sometimes from standing stationary. So my question is: How often should I do leg machine exercises in the gym ( for e.g leg press ) in a week? Daily? Once every 2 days? I'm not sure so please share your knowledge with me.