How long do we have to file an amended Federal return?

[scm84]

My son was told by Intuit TT to check his 2020 returns for a possible error and file an amended return if needed.  I checked this for him and completed the amended return, but since it has to be mailed in it requires his signature and he's out of state for several months.  How long does he have to file the amended 2020 return?

[VolvoGirl]

3 years.

[scm84]

Thank you.