Online payments to vendors usually post within 24 hours, or on the next business day.
Note: Even though the funds have left your account, the vendor may still not have received the payment. For example, if the payment processor has to create a paper check, it may take up to four business days for the vendor to get it.
However, you can time a transaction to take place when you want, and so manage your cash flow. For example, if you have a payment due on the first of each month, you could send your payment request several days early, with instructions to post on the first of the next month.