I missed a W-2. I followed the app instructions, printed it, and sent it. When can I expect to receive my refund for the missed W-2?

It is suggested that it be mailed certified with return receipt (or other tracking service) to verify that the IRS receives it. That is the only proof of mailing that the IRS will accept.

-- Amended returns can only be mailed - allow 8-12 weeks - can take up to 16 weeks (4 months) for processing.

***************************************NOTE***********************************************
Amended returns can take up to 16 weeks (4 months) to process in normal times but due the the Corona virus shutdown the IRS is not even opening mailed returns at this time and just storing them so the 16 weeks will not even start until the IRS returns to normal - whenever that will be - and there will be a huge backup of mail.
https://www.irs.gov/newsroom/irs-operations-during-covid-19-mission-critical-functions-continue
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See this TurboTax FAQ for help with amending:

https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

You can check the status of your amended return here but allow 3 weeks after filing for it to show up:

https://www.irs.gov/filing/wheres-my-amended-return

If you printed and mailed the federal amended tax return, Form 1040X, then use this IRS website for the status of the amended tax return - https://www.irs.gov/filing/wheres-my-amended-return

IRS amended tax return FAQ's - https://www.irs.gov/filing/amended-return-frequently-asked-questions

In normal years the status should be available about 3 weeks after mailing the return.  This is Not a normal year.

See this IRS website - https://www.irs.gov/newsroom/irs-operations-during-covid-19-mission-critical-functions-continue

The IRS is not currently able to process individual paper tax returns. If you already have filed via paper but it has not yet been processed, do not file a second tax return or write to the IRS to inquire about the status of your return or your economic impact payment. Paper returns will be processed   once processing centers are able to reopen.