When you love the gym, this is not a question, so for those who are at the stage of not really loving having to fit exercise into their life, any amount of time is enough. If your fitness routine in the early months is no more than 20 minutes a time, that's okay. It's all about building habits that will last a lifetime. Your fitness routine may change year to year, but the habit of fitting exercise into your 'non-negotiables' is forever.