How often can you need to do workouts that increase metabolism to keep it up?

Add weight training: Aerobic (running, biking, swimming, elliptical, etc...) exercise increases metabolism only for the duration of that exercise. If you want to keep your metabolism going after your workout has stopped you need to add weight lifting to your program. Not light dumbell exercises, but real lifts like squat, deadlift, heavy kettlebells, etc... Work with a trainer and ramp up slowly.