RIN is a Bell terminology, and each aircraft type has components which are lifed based on either hours or RIN cycles. A mast may have a life of perhaps 5000 hours or x amount of RINs, this is a fatigue cycle calculation, and the formula is provided in the relevant aircraft models ASB's and Information Letters and the Chapter 4 of the relevant Maintenance Manual. The component (In this case the mast) could, depending on the nature of your flying be due for retirement prior to the hour figure being reached, if you are heavily involved in external load lifting for example. Bell assign this limit, and it is the responsibility of the pilot to accuratly record torque events, landings, hoist cycles etc, which are stipulated by the manufacturer. Your maintenance facility should be able to explain this specifically for the aircraft type you are flying, as it is they who should apply the formula to your aircraft, and they should advise you if you are approaching a retirement limit, either hour or RIN based.