From racing, nothing to do with aircraft, We check to see how good a job we did building the motor with the leak down and use that as our base line. After each race we do it again and form trends. We run it on the dyno at different leak downs and see how much it matters. In my experiance when it starts changing rapidly it is time to adress the problems. Valves, as they begin to leak, don't cool as well and get worse. Rings, as they began to leak burn the oil coating the top of the cylinder and the condition deterioates rapidly. So in my opinion if you do a leak down every time you change your oil, you will be able to spot a downward trend. In racing we always found it less expensive to rebuild sooner rather than later.
In the higher levels of racing we would ring and valve it every race (300) miles and pull it down to the crank every other race. When running nitro even if nothing went wrong the engine would last about 35 miles wide open. Usualy we didn't make that TBO. Thank you, Vance