Jittered exponential backoff. You don't want the whole herd to come back at the same time, you have to add timing jitter to the clients.

Don’t forget jitter!

Exponential is also overkill (even with jitter as others have mentioned).

I seem to remember there was a "you failed 5 PIN entries in a row, please wait 500000 seconds before you retry" on Apple phones. So, you probably also want a sensible max... which makes exponential a bit pointless. Just do a basic fixed delay + (large, e.g. 0.5 x the delay) jitter and you'll be fine for most things. You can add a bit of cumulative delay if it's really costly to do retries.