From information previously posted on the crash thread, the Service Life Limit used to be 5000FH and the TBO 2000FH but the TBO is said to have been reduced to 1500FH. With a rotor speed taken as 265 rpm and the 2nd stage sun gear rotating at 820 rpm, I reckon the 51 tooth planet gear should ‘rotate’ at approximately 1013 rpm (about its own centre). To achieve the original 5000FH SLL a planet gear would have to rotate approximately 304 million times.
To answer the question asked though, in a ‘lifetime’ the planet gear may rotate anything between 607,800 and 304,000,000 times. That 'lifetime' would be based upon the directive to manually inspect all EC225 MGB magnetic chip detectors every 10 flying hours applicable, it would seem, to even brand new gears. It should be noted for fatigue considerations that cyclic bending stresses occur twice per rev of the planet gear.