You can buy 1 year or 2 year coverage from the off if I recall. If you purchased 1 year and haven’t used it, you can convert it into the 2 year plan.
Never had a plan expire as yet as sold my Mini 2 before it expired and purchased the 2 year on the Mini 3 Pro, so not had to deal with it, but the wording of the FAQ suggests you can’t keep buying it. I guess due to wear and tear and age of product at that point.
"If your DJI Care Refresh (1-Year Plan) has not yet expired, and there is at least one product replacement left, then you can purchase DJI Care Refresh +, which extends your replacement service for one more year as well as prolonging the warranty period of your product.
Since DJI Care Refresh + provides a second year of after-sales services, DJI Care Refresh + cannot be purchased if you already purchased the 2-Year Plan of DJI Care Refresh.
DJI Care Refresh cannot be purchased repeatedly. Each DJI product can only be bound to DJI Care Refresh once. After using the service provided by DJI Care Refresh, if the product’s serial number is changed, the provided product will automatically bind to the original product’s DJI Care Refresh, and the original service and warranty period continues. Therefore, the original unit will no longer have a DJI Care Refresh Plan. Please do not re-purchase DJI Care Refresh for the replacement unit."