When you send money to someone, the amount is immediately debited from your credit card or balance, to ensure that you can send it to the recipient. Lydia is a trusted third party.
If your contact already uses Lydia, the amount is instantly credited to their balance. If they don't use Lydia, they have 7 days to accept the money (either by typing their IBAN, or by signing up to Lydia).
If they don't accept your transfer within 7 days, the money will be automatically credited to your balance ,or bank account if you used your card to pay. In this case, you will see it on your bank account statement within 3 working days.