It depends. Traditionally, ashtanga is considered a daily practice, with six days of practice per week and rest on moon days. That being said, it is not about following a rule, but about finding out what works for you. Although it is a fact that you will benefit most from a regular practice, it is equally important to make the practice fit your life, not your life the practice.
As a general guideline, you can try to practice three to five times per week. But even one or two times per week can already make a big difference. Note that it is always better to practice more often but shorter, than to do a long, intense practice less often or even on an irregular basis. Consistency is key.