I did it at 39 but read somewhere that once you finish the main game and everything scales up to lvl 50 the lock on the treasure lvl is removed too allowing me to get lvl 50 loot.
Target 40k - Hit 18/12/2010
Target 50k - Hit offline date unknown
Target 60k - Hit 19/10/2014 (Brain Drain - Borderlands TPS)
Bioshock Infinite: Clash in the Clouds - Blue Ribbon Guide