It says at the payment site it can take up to 5 days:
"It typically takes up to 5 business days following the receipt of your payment to restore your Available Credit for future purchases."
Although, I made a payment about 24 hours ago, it shows as Paid at my Bank and both Last Payment that amount and with my full Available Credit under "Account Summary" at OpenSkyCC, but under Payment History does not show this most recent payment yet and under "Pending Payment" it says Processing.
So it appears to be pretty quick, but I'd just expect the 5 days to process to be safe.