Usually, when credit’s headed your way, it can take up to two hours before it arrives in your account. If you’re waiting on credit from Refer a Friend, make sure you (and your friend!) have an active payment method and have made a successful charge since the referral (minimum of $1)!
Credit will expire after a year, so make sure to start a charge before it disappears! You can see the expiry date in the ‘Credits’ section of your app.