i think a good rule of thumb (mind you this is in a perfect world, and will vary based on title, make, ball times, environment, types of players, and countless other factors, so take these as estimates), but around every 1000 games you should... change out coil sleeves on all mechs, replace worn flipper rear coil stops (always both bottom flippers together, to maintain alignment and stroke), tighten bumber posts and rails from above and below PF, wipe down and check plungers on all mechs, check alignment on plunger, replace pinballs, clean bill validator reader (if applicable), plus probably many other details i'm probably forgetting. things like cleaning, changing rubbers and light bulbs should be done more often than that, as needed. and as others have said, don't leave until you credit up and play a full game. make sure you didn't leave a screwdriver sitting in the middle of the playfield.