2 years and 1 day of 4 years/51% of a term as per the 22nd Amendment.

Section 1. No person shall be elected to the office of the President more than twice, and no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of the President more than once. But this Article shall not apply to any person holding the office of President when this Article was proposed by the Congress, and shall not prevent any person who may be holding the office of President, or acting as President, during the term within which this Article becomes operative from holding the office of President or acting as President during the remainder of such term.

The important bit:

and no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of the President more than once

So assuming Joe's term is precisely 4 years than the day after the 2 year mark he can fall down some stairs again step down and Kamala can serve out the remaining 1 year and 354 days then still go on to run for 2 whole terms after.