Once a transaction is made by a customer, the information is immediately sent to the blockchain network. The speed of transaction processing is directly dependent on the chosen cryptocurrency, network congestion, and the fee of the chosen network.
It typically takes a few minutes to twenty minutes for a sufficient number of transaction confirmations to accumulate on the blockchain and then credit the recipient's balance.
If the funds have been sent in full and the transaction has not been confirmed before the invoice expires, the payment will still be credited after two network confirmations.