I too was wondering about this. I was not able to get a straight answer from anyone. I didnt want to ruin my cylinder by waiting too long, and didnt want to tear down my first new virgin bike too early. What most people said was " when you notice a lack of power" or "when it kicks over too easy" Some people also rely on compressin readings, I have seen way too many guys get tripped up over faulty compression testers. A compression tester will vary its reading, depending on the spring pressure of the lower schrader valve. (word to the wise)
I tore my upper end apart after I had put two hundred gallons of fuel through it. What I found was a perfect cylinder, perfect piston, excessive oil film everywhere and the piston rings gooey and sticking in the piston grooves. I decided at that point that I didnt need that much oil, so I discontinued 32 to 1 and switched to 40 to 1, I may even switch to 50 to 1 next year. I washed the power valves,beaded the ring grooves, replaced the rings and went back to riding with no more worries. The cylinder head was never removed from the cylinder and I still have the virgin head gasket intact.
It is very easy to remove the cylinder and piston, the only point of concern is the adjustment of the kips lever. Elsewhere on this site is information on how to shim the valves from overclosing using a ball bearing, once you get that figured out and how to adjust the lever, this job is super simple.
Hope this helps, John