Originally Posted by Moonwolf

You have to keep a record of endorsements for THREE years.
- 61.189 Flight instructor records.

(a) A flight instructor must sign the logbook of each person to whom
that instructor has given flight training or ground training.
(b) A flight instructor must maintain a record in a logbook or a
separate document that contains the following:
(1) The name of each person whose logbook or student pilot
certificate that instructor has endorsed for solo flight privileges, and
the date of the endorsement; and
(2) The name of each person that instructor has endorsed for a
knowledge test or practical test, and the record shall also indicate the
kind of test, the date, and the results.
(c) Each flight instructor must retain the records required by this
section for at least 3 years.

After the get their PVT, I'm not sure if there is a deadline of when the responsibility is cut...probably not, but if its been a few years I'm sure less of it would fall on too the instructor.