@CharlieCrackle, generally replacements with the standard warranty are Next Business Day, but the official line is they ship within one business day on a best effort basis. It’s those last 5 words which are the gotcha.
If you need a more solid replacement agreement then you need to look at Meraki Now, which is somewhat equivalent to Cisco SmartNet. I believe it’s only then do these devices get factored into Cisco’s sparing model to ensure there is adequate supply in local depots.