As long as you don't uncheck the "Instant withdrawal" box your withdrawal will be processed immediately. If you do uncheck the box then your withdrawal will be queued and processed within 24 hours.
If queuing your withdrawal was a mistake you can always cancel the withdrawal from your withdrawal history at https://bustadice.com/account/withdraw and request it again as an immediate withdrawal.
Do non-immediate withdrawals get included and tagged onto the next person that does an immediate withdrawal, if not, why not do this?
Btw, on bustadice the wording is correct: "processed within 24 hours", but on bustabit I think the wording is incorrect, it says "at least 24 hours" which I'm guessing is not your intention to have a required minimum of 24 hours, and you actually mean "within or up to 24 hours"