If it was me in your situation I would do it every year. I think you'll find the top surfaces get most of the guck on it. Doing it reasonably often makes it easier to do.
Turn the bar inside out often, I tear it into two pieces and it does the car twice.
Use a goodly amount of lubricant on it, and only do it after a good car wash, no need to get grit in the bar, if you drop the bar on the ground dump it for the same reason.
I've only used Mothers detailing spray on mine, a little washing up liquid in water works as well I guess.