I have all of mine, back to the mid 1960s. Each return in a manila envelope with all supporting documents. And my check registers but they only go back to 1984. I have been a researcher and historian for some time and at this point, I simply cannot bear to discard them. Never throw away history!
Certain documents need to be retained indefinitely.
E.g., documents about purchasing your house and expenses spent on it. So when you go to sell your house you can reduce the capital gains tax on it. (Note the current exclusion limit is $250k individual, $500k couple. Easily reached in some markets.) There’s a lot of messy stuff if you want to do it right. Insurance payments, depreciation, etc. And if you were taking deductions for a home office that’s a whole 'nother area of documentation you need.
And investments in general. Records on how much you paid originally, etc.
Just a couple weeks ago I cleaned out the file drawer that has our tax returns. Everything older than 10 years got shredded, which was as far back as 1994 or 95. (There was also one return of mine and one of my wife’s in the file from around 1986 or 88, before we were married. Not sure what we were saving those for.)
I just checked; I have them back to 1998 in the proprietary format used by TurboTax, and back to 2003 in PDF format. For the most recent years, there never was a printed paper copy of the return, as I filed electronically.
I don’t believe you need to keep the supporting documentation for any real purpose past 7 years of the date they were filled, but you should perhaps always save the returns themselves. One situation comes to mind, is if when you retire you need to show what years your traditional IRA contributions were non-deductible.
About 10 years ago, I set-up a file server and got a document scanner. So over the years, most things which were paper records are scanned into the computer. So we are in the practice of scanning in paper records, statements, etc., constantly and then shred them. It is nice to finally notice we not only won’t be buying anymore filling cabinets, there are empty drawers in the ones we have. I’m hoping to actually get rid of some of them.