I do it every few trips out or every time if I have the available time. Having a sticky board is NO FUN at all. Once a season won't cut it for sure. I wouldn't even personally push it to 6 days myself but I guess that depends on how many runs you get a day etc. as well.
You'll know when it needs it bad because the board will be a flat out pain in the ass to ride. From there, it's your preference