Also keep in mind that engines with lower rpm's do not have the same material rods as the faster ones. Novarossi did number them in strength and currently I have seen R2 up to R7.
The R2 is placed in a Basic N21-3T and from own experience I can say it won't last long if you tune up the engine to the performance of the high priced competition models. I have seen R3 and R4 rods in offroads and the R5 and R7 are used in onroads, the R6 is a special model for JP.
Changing rods is cost effective on 600+ dollar priced engines, waiting until it breakes will cost more....