Once every 3 years
I would say depending on the severity of the winter and whether or not the roof was shoveled during the snow. I would suggest yearly inspections if you get hit badly and every 3 years if you don’t have bad snow. Rubber roofs can easily be patched and fixed when an issue is caught early on and it can save you a lot of money to patch as opposed to replacing if water gets in and wets the insulation.