Meds exist <50yrs !
Suppression of the immune system with various drugs was really only developed in the 1960's and after. So, few patients have been on these types of drugs for more than 40 yrs.
But there is no specific clock that limits their use. As with any medication, use balances the good hoped for, with the bad it might cause. For txp recipients, there is no alternative since organ loss = death (or dailysis).