We recommend doing at least once a week. If flexibility is more of a problem with you then twice a week would be more beneficial. We usually recommend that you schedule your yoga session the day after you workout. By doing this it will help you recover quicker getting you back into the gym for your next workout.
By adding yoga to your weekly training you will begin to listen to your body and get to understand the messages of your body. You begin to learn where you are strong or weak, tight or flexible, or simply holding on to stress in certain parts of your body which can result in injury.