Depending on the payee, bill payments will be either be made electronically or by a physical, mailed check. (The method used is shown in your transaction history as "Delivery method").
Electronic payments are completed in 2 business days. Some billers may not show the payment right away, but once they do it should be credited as of the delivery date.
The mailed check(s) sent for bill payments are withdrawn from your account 5 business days prior to the due date to ensure delivery on time. Keep in mind that check processing time may vary.