28 C.F.R. § 345.55
(a) Except as provided in paragraph (b) of this section, an inmate earns longevity pay raises after 18 months spent in FPI work status regardless of whether or not the work was continuous. The service may have occurred in one or more FPI factories or shops. An inmate qualifies for longevity pay raises as provided in the table below:
Length of Service With FPI After 18 months of service and payable in the 19th month After 30 months of service and payable in the 31st month After 42 months of service and payable in the 43rd month After 60 months of service and payable in the 61st month After 84 months of service (& more) and payable in the 85th month
Longevity pay allowances shall be added after the wages for each actual hour in pay status have been properly computed.
(b) Exceptions.