GRP is good stuff. The life of the rod is going to depend on the rod-ratio, what your swinging, and the end user. I wouldn't worry at 250 passes unless the ratio is way short and/or you make a habit of beating on the motor cold. But, 500-600 passes are not equal in all applications either.
As for the other comments, I have seen an aluminum crank and plastic connecting rods.

Good Luck