One of my friends had enrolled in IGNOU this year and is presently giving exams. He has been to his study centre to get books, submit assignments, clarify doubts, and at last to sit for the exams. 

If you keep yourself updated about these essential things like the assignments, etc. Then you have no need to visit the study centre frequently. If necessary, visiting it once a month will be fine.

If your study centre responds well then you can call them on the particular days they attend calls.

But if you are a new candidate and are not familiar with the process, then it is recommended that you visit it in alternate weeks. This will help you get updated about receiving books, Roll no., admit card, etc.