Actually the average production length of SPECIFIC models lasts about 20 years. For example the 737-200 was only in production for about 20 years, and then was shut down, but other variants of the 737 still remained in production.
I think what he means is when will production of the 757-200 itself end? Production of the 753 should not end till about 2014. But the 752 has been around for 20 years, and so is, by Boeing's normal standards, due for shutdown or replacement.
IF a 757-200X or LR comes into existence, -200 production will end, and all production will shift over to the X or LR product, which if it were started today should last until 2022.