I did a search before asking. In the last couple of months I did A LOT of gas stations (mystery/reveal). I'm fairly new so I don't know if they cycle through,if so when? I only see a couple of gas stations now.
Gas stations generally are done quarterly, although some are done monthly. Quarters generally run Jan - Mar, Apr - Jun, Jul - Sep, and Oct - Dec. As the quarter nears the end, the fees typically go up (sometimes VERY high) and the number available goes down. Don't worry, you'll see a LOT of new shops in early July, but the fees often tend to be low. As the quarter progresses, the fees go up as the MSC tries to fill the remaining shops before the end of the quarter.