1) when there is to much play, with a new rod by hand you must feel if it is the rod or the crankpin.
2) after an amount of liters fuel used. There is also a non-visable wear in the rod on molecule level, the material is getting tired and so loosing strenth, thats why to change it from time to time.
This periode is mostly mentioned in the manual.
Speed is only fiction but a 130+km/h MRS4 is for real.