How long do I keep Form 8879?

Short answer: three years from the return due date or the IRS received date, whichever is later.

For e-signed 8879s, the signing record (signed form, time, IP, verification result, signing method) must be available to the IRS on request.
Records must be tamper-proof, indexed and printable as legible copies.
Keep the Submission ID (or the acknowledgment showing it) with the 8879.
Don't rely only on your signing tool: download the signed PDF and certificate into your own files.

Every SignLucid signed PDF ends with a certificate of completion and SHA-256 fingerprints.