They are all different. Some are monthly, some are quarterly, some are just a one time random. There are so many in my area that I do not keep track of how often they come back.
If you want some easy ones to start off with, start with the ones on Market Force that say "mystery shop". They are the easiest and are the first step on the regular gas stations. Once you get used to those, then you do the ones that say "mystery shop" AND "audit" (which have additional pictures added to the form for the mystery shop). Once you learn this process, then you should be ready. However, every brand is slightly different so save those documents to your tablet and use while doing the revealed audit.
Shopping across Indiana but mostly around Indianapolis.