I call my mom about every day- now that she lives in town and it's not a LD call! When she lived in CA, about every other day. I would say that besides DH, my mom is my best friend. I don't know what I would do without her! I am 33, she's 58. She retired last year, 8 months early, because of me- she was my support system while DH was in Iraq and I was pregnant with my 3rd baby.
However, if I was not as close to my mother, I would think twice a week would be more than adequate for calling. DH talks to his mother about once a week; sometimes more, sometimes less.