Received a CP09 for stating I could have claimed EITC but didn't. I mailed a day or 2 later, when should I expect a refund?

IRS website for the CP09 Notice you received - https://www.irs.gov/individuals/understanding-your-cp09-notice

If I'm eligible for the credit, when will I receive my refund?

We’ll mail you a refund check in 6-8 weeks if you don’t owe tax or other debts we’re required to collect. If we deny the credit, we’ll send you a letter of explanation. If you don't hear from us after 8 weeks, you can call our toll-free line at 800-829-0922 to check the status.

If you used TurboTax to complete and file your tax return and you were eligible for the EIC, TurboTax would have automatically entered the credit on your tax return.

Look at your federal tax return, Form 1040 on Line 27.  If there is an amount entered this would have been included in your federal tax refund or it would have reduced the amount of taxes owed.