Leah Libresco, writing at Five Thirty-Eight, checked out how oftem moms would like their kids to call, and how often kids think they should talk with Mom.
My average is once a week, but sometimes more if the occasion presents itself. Or she needs help with her computer -- although those conversations tends to be limited in scope: "No, you didn't break it when you dropped the mouse." My sister talks with her more often. Many weeks, although there might be only one phone conversation, there's a few e-mails. Some of which also involve helping with the computer. Or her iPad.
Whether you call her as often as she likes, as often as you like, or somewhere in between, you'd probably better remember to call her tomorrow. Greeting-card companies may have invented the holiday or they may not, but it's for real now and if she has to call you on Mother's Day..., well, in the words of Stanley Craver, "Bad things, man."