The phrase "accumulation period" refers to a specific period of time during which someone is saving or gathering something little by little. It means saving or collecting slowly over time. Full definition
We do not yet know much about accumulation periods shorter than 1 - day. (pacificclimate.org)
Synoptic monthly means are averages over days in the month for a particular synoptic time eg 12 UTC, or in the case of accumulations, for only a particular accumulation period within the day eg 06 to 12 UTC. (reanalyses.org)
Others will save, but pay fees far too high for the privilege, possibly costing them half their pension over a 40 - year accumulation period.» (moneysense.ca)