There is no golden rule but I'd say at least once a week. Less than this is probably too infrequent.
Washing your hair is important because:
If wet hair is so great why NOT wash or co-wash daily?
That said, two or three times is plenty even if you go to the gym daily.
Some people co-wash daily, however, on my 4B/4C hair which takes me about 2.5 hours to detangle, condition and moisturise, daily simply wouldn't work. You are the master of your hair so start of with a weekly wash and adjust that if you think it's not enough.
The Science of Black Hair, pages 26, 43-4, 99
How often do you really need to shampoo? (webmd)