I have an OS .12CV-RX that has 5 gallons (approx) thorugh it. It's loosing compression and can only really be started on a starter box since there aren't enough rev in a single yank of the pull start and I can't yank it as fast as a starterbox can turn it. I also have an OS .15 CV-R that has almost 5 gallons through it and it still has great compression.
It all depends on how you run it (Rich or Lean) over heating and fuel percentage also have a play in it