Forever.  Here's why.

A friend bought a business from his uncle 30+ years ago. They are on a state line and do business in Illinois and Indiana.  They have been meticulous on taxes.  Over more than 50 years there's been I think 5 different accountants doing taxes.  From independent to a multi state firm.

A few years ago Indiana audited their sale tax.  Came out so close to perfect there was no problem.  Auditor said "All I need is a copy of your Indiana income tax filing and I'm done"  I suspect that was the real reason for the audit.

They replied "Indiana?  We are an Illinois company."  Turns out if you do business in Indiana you have to file Indiana income tax.  In all the years with all those accountants nobody ever filed or even mentioned it.

How is that relevant?  Because it was considered fraud and they could have gone back to day 1 and demanded all Indiana income taxes be paid.  They limited it to a few years.  He bought the state of Indiana a very nice pickup that year

That's why I keep records forever.  I do my best but in case someone messed up I want to be ready