But guess what? Congress can pass anything they want and make it retroactive as long as the President signs it.
How the courts interpret that is a different matter.
Article V that has the procedure, is pretty straight forward where there was no time limited required -
Article V
The Congress, whenever two thirds of both houses shall deem it necessary, shall propose amendments to this Constitution, or, on the application of the legislatures of two thirds of the several states, shall call a convention for proposing amendments, which, in either case, shall be valid to all intents and purposes, as part of this Constitution, when ratified by the legislatures of three fourths of the several states, or by conventions in three fourths thereof, as the one or the other mode of ratification may be proposed by the Congress; provided that no amendment which may be made prior to the year one thousand eight hundred and eight shall in any manner affect the first and fourth clauses in the ninth section of the first article; and that no state, without its consent, shall be deprived of its equal suffrage in the Senate.
https://www.law.cornell.edu/constitution/articlev
Except what happened, when the original ERA passed Congress, it included a legislative timeline, but one not incorporated in the text of the actual amendment, and that is where the opinions diverge on what happens next. In fact, the 27th Amendment, that suddenly bubbled up after a couple hundred years, ended up going into effect in the '90s when it was found the process had started but hadn't completed, and then a few more states signed on when they discovered it, and Congress went on and accepted the validity of the previous ratifications through new resolutions, and the 27th became final.
The CRS has a good write-up on this here that I remember reading a couple years ago -
https://crsreports.congress.gov/product/pdf/R/R42979 (PDF)
= new reply since forum marked as read