Until at least three years after you file the return reporting the reimbursement — which, if you are delaying reimbursement by decades, means keeping them for decades.
The standard IRS record-retention guidance is three years from the date you filed the return in question, extended to six years if income was substantially understated.
But that clock doesn't start when you pay the bill. It starts when you file the return that reports the reimbursement. Under the shoebox strategy, that could be forty years after the expense.
Practical rule: keep the receipt until at least three years after you file the return in which you claim it. For an expense you plan to reimburse in retirement, that means keeping it for the entire holding period plus three years.
Which is a long time for a thermal-paper CVS receipt that fades to blank in eighteen months. Digitize early, store somewhere you control, and keep an export you can move.
More detail: How Long Do You Need to Keep HSA Receipts?