I think its once every 24hrs, or when your device is on WiFi.
I go to bed every night between 1-2am, this is around the time I finally plug in my iPhone to charge, and so I notice my sync pattern is generally between 2-3am every day.
My iPad on the other hand I can go days without charging, so I notice that it generally begins to back-up about an hour after I finally plug it in to charge.