How long can you hold a check from someone that died?

It Depends...

A check is valid only for 90 days (180 in some countries), so you cannot hold a check beyond this date

plus

Since the check issuing person is dead, the check should have been issued to you before the date he/she died. so if they issued you a check on April 1st and died on May 1st, you have until June 30 to cash it, after which the check will become stale or useless