Depends on the funding source used by the sender.
PayPal Account Balance - almost right away.
Credit Card - amost right away.
Bank Account - anywhere from 2 hours to 8 days plus. (depends on the sender's country)
Other things to toss into the mix. Are both the sender's account and receiver's account "verified" and have a clean history of tranactions (meaning there's no trail of disputes, claims or refunds). If yes, this reduces holds and reviews on transaction which can add time.
