When did you last have a ....
Excerpt below is from a study published in the International Journal of Yoga, the full link is below.
While no concrete guidelines exist regarding the frequency of practice, the more you practice the more you benefit. Yoga is a personalized practice and as such, frequency and duration are personal questions with individual answers. Practice should happen with wisdom and should be modified to meet individual needs and goals. Individuals should practice as often as possible, especially in the beginning. The length of the induction phase will vary depending on an individual's initial level of fitness and health status; the more difficult yoga is for someone in the beginning the more their body needs it.