About 53 hours. I took my time and never rushed in order to save ammos.
Besides, what's the point of rushing through a good game? ^_^
All my played games have exactly 1000G.
1st 1000G: Batman Arkham Asylum (PC)
1st Xbox360 game: Trusty Bell Chopin no Yume / Eternal Sonata
1st Xbox One game: Child of Light
1st Xbox360 player to obtain Obsessive Compulsive (collect all loots in a single chapter) in Thief on 25/2/2014 at Trueachievements.org