If you are flying under Part 91 – You need to check whether the insurance company or company you are flying for has a required renewal timeframe. (The FAA does not have a specific time indicated for renewals.) There is no FAR that sets a requirement for RVSM Recurrency for Part 91 operations. Under the LOA, the FAA expects that the person designated as the responsible individual will ensure that all pilots are qualified to operate the aircraft. Depending on the relationship with your POI you may or may not have training requirements specified in your agreement or operations manual. The primary driver is more likely company policy, personal standards or insurance considerations. LOAs used to expire after 24 months and again it was between the operator and the POI what was required to renew the LOA. LOAs are aircraft (tail number) specific and now must be renewed only when any one of three things happen: (1) change of equipment, (2) change of ownership, and (3) change of responsible individual. Again, it will be between you and the POI as to what will be required to issue a new LOA if any of those things change.