It depends - I've had one client recently (last year) who had omitted to report his 401K withdrawal on his 2015 tax return and he just received a letter last March, 2020. However, I've also seen instances where folks get a letter in August for the tax year just passed. If you already know to expect a letter and know why, it is in your best interest to file an amended as soon as the IRS fully processes your return. For any years that you are required to mail in the amended return, be sure to send it out Certified Mail / Return Receipt Requested as this is the only proof you will have that you've filed it, when you mailed it, and the fact that the IRS has received it (they will sign the "green card" and then the USPS mails this back to you - this is the "return receipt" part).
