How long do business records need to be kept?

It depends on the type of record.

Proof of the cost of an asset should be kept as long as you own the asset (purchase, modifications, major repairs) but routine upkeep and maintenance records aren't needed for tax purposes.

Employee records may need to be kept for certain time periods based on labor laws, but for tax records you'll want to keep Form W-2 records for at least 5 to 7 years.

Documentation used to fill out your tax returns should be kept for at least 5 to 7 years in case of audit.

Investment information should be kept as long as you own the investment (shows proof of basis, such as reinvested dividends, etc). Remember that monthly statements may not be needed if an annual statement reconciles or shows the activity for the year.

There's probably other documents I didn't cover in this general explanation. If in doubt ask a tax professional regarding your personal situation.