I believe it was 7 years or less.
There was always a lot of complaints back when parts for the 426 wasn't readily available, even when only a couple of years old.
I was in a Mopar dealer ship in 1980 during the Chrysler/economy flop, and they had NOS quarters and fenders they were returning to Ma Mopar for a credit.
They had Challenger quarters and 71 Cuda front fenders for $ 100.00 a piece.