Yoga can be done in various ways. You can be a hard core Yogi, and do Yoga practice for hours every day or you can just practice a few times a week. It off course depends on how much free time you have or how serious you are with your practice.Most Yogis try to keep a regular practice to build up the energy and cultivate more their inner force to gain better effects.
My personal advice on this would be to try and get a daily practice that you enjoy doing and can easily manage to do and stick to it, this way whenever you find you have extra time - that day you can add more to your practice and enjoy deeper effects. If not, then you can always fall back to your basic practice which will keep you strong and focused.
Good luck with your practice!
