According to the manufacturers on a 125 they suggest a new top end every 10 hours. Unless you are freaking Grant Langston, then it is overkill IMO.
The guys that need new top ends every 10 hours has thousands of dollars riding on a single race, so a $100 top-end is basically an "operating cost".
Unless you ride HARD and OFTEN you are not going to notice a 7% decrease in power on your bike over the life of a top-end. For the weekend-spode to go around doing their top-ends every 8-10 hours is a only blessing to the guys at the parts counter IMO. It is a waste of time and cash if you ask me. For a two year old bike to have gone through only 2 set of tires, but had 10 - 12 top-ends done on it is foolishness. I have spoken with a mechanic that toured with the pros and he would save the top-ends that were changed out every 4 races and run them on his bike because they still had plenty of life left in them.
If you only ride 2-3 times a month then you can probably get by with doing a top end on a 125 every 4-6 months providing you do the required air filter and crank-case oil servicing. When your bike becomes hard to start, you might have waited too long, though!:D
BTW - if your powervalve needs cleaning every 10 hours, you might need to re-jet (or you are really getting in more than 10 hrs of riding time!). I just yanked the top-end off of my YZ after 5 months of operation and the piston is in A-1 shape (almost zero carbon build-up) and the powervalve was very clean. The cylinder bore was shiny and still looked like it was just honed. I only needed to run the cylinder under the parts cleaner a couple of times for the powervalve to look like new again.
Disclaimer - when it comes to the care and feeding of my bike I am pretty anal about this stuff. I change the oil every 3 rides (with the good stuff like Solkolene or Maxima) and the air filter gets cleaned every 2-3 rides. I am also cost-conscious though and spend money when warranted. I can not justify popping a $100 top-end in my bike every 10 hours of operation when I only ride 2-4 times a month (and one of those times is a TORO race).
Sorry for the long post - and of course, YMMV...
-jeffd