To be precise, it's 3 years from the return due date or IRS received date, whichever is later.  Below are the texts that come straight from the filing instructions.

Important Notes for EROs
• Don’t send Form 8879 to the IRS unless requested to do so. Retain the completed Form 8879 for 3 years from the return due date or IRS received date, whichever is later. Form 8879 may be retained electronically in accordance with the recordkeeping guidelines in Rev. Proc. 97-22, which is on page 9 of Internal Revenue Bulletin 1997-13 at www.irs.gov/pub/irs-irbs/irb97-13.pdf.