I've gotten as many as 25-30+ on a MC-9 plug with the engine tuned correctly. If you've gotten to 10 you should be able to continue on with it. You could pull it out once in awhile and see if the element is getting beat out of shape. Should look normal, else change it. Generally, the only indications I've seen for a plug going south is the idle starts to act up or it won't start due to a broken element wire. Many folks like to change them just to be safe but I've found more often than not, something will happen and the plug will get blown whether its new or not. By "something will happen" I mean you got it too lean for any number of reasons: the head bolts loosen, filter gets clogged, or something like a flake of exhaust o-ring goes thru the engine, etc. BTW, I've had flakes of exhaust o-ring end up in in the tanks from going thru the pipe pressure tap to the tanks and then back to the mixture control. So make sure you use filters.