How soon after my federal return is accepted can I go online and pay by credit card

[DoninGA]

You can do so at any time after the return is accepted by the IRS.

Go to this IRS payment website for paying the federal taxes owed by credit or debit card - https://www.irs.gov/payments/pay-your-taxes-by-debit-or-credit-card

[VolvoGirl]

Right away.  Don't wait.  You can pay the IRS directly at
https://www.irs.gov/payments