Does the 21 days start from the time you file your taxes or from the time the IRS starts processing returns?

It starts from the date your return is accepted by the IRS.

@TurboTaxEdwardLO my return was accepted on 2/11, would my 21 days start on 2/11 or 2/12? thanks!

21 days is only an estimate.
<a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-pdf/p2043en.pdf">https://www.irs.gov/pub/irs-pdf/p2043en.pdf</a>

@SweetieJean thanks a lot!