by law for how long a tax preparer keep original form 8879

Yes... three years is the time requirement for F 8879 retention.

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.

It may depend upon your state. Here is the California requirement from (of all things) the instructions for the 8879

The general California statute of limitations is the later of four years from
the due date of the tax return or four years from the date the tax return
is filed. Exception: An extended statute of limitations period may apply if
the taxpayer’s California or federal tax returns are related to or subject to
a California or a federal audit.