I get a lot of emails from musicians who want to know how often they should be practicing if they want to get serious about their music career. Really, the answer is that you should practice as often as you can. Regular rehearsals are especially important if you're part of a group, since playing together often is the only way you can get to sound tight and develop an instinct about each other's playing. Practicing music often can also spark your creativity so you keep your songwriting up.
However, when music is not your full time job, real life often prevents you from getting as much practice as you would like. The best way to cope with this is to make a practice schedule - say, once or twice a week, whatever you can manage - and then stick to it. Having a schedule, especially if you're part of a band, will help ensure that playing remains a priority and doesn't get put off and put off until you haven't practiced in recent memory.
Of course, playing when you get a chance between practices will help you a lot as well. If you're not busy, pick up your instrument and get some rehearsal time in. These mini practices will make your scheduled practices that much more fruitful - which is extremely important when your time is limited.