Flip the engine by hand very briskly to get a good compression reading. Several flips will do the job. As far as numbers go, we don't have any. O.S. doesn't provide any. If your engine is running well, then check its compression and see what number you get. Then a check from time-to-time will let you know if it's falling off. How the engine's running is a very good tell-tale to determine if you need to check compression.