How long does breakdown of collagen etc. take for sirloin cooked sous vide?
Sirloin steak doesn't have a huge amount of collegen, which is why it's traditionally cooked over high heat for as short a period of time as possible. If I were you I'd be more worried about the meat become unpleasantly mushy if left in the bath for too long. I would just use the Douglas Baldwin tables for bringing the meat up to the required temperature:

Heating Time from 41°F (5°C) to 1°F (0.5°C) Less Than the Water Bath’s Temperature

Thickness   Slab-like   Cylinder-like   Sphere-like
  15 mm       35 min       18 min          13 min
  20 mm       50 min       30 min          20 min
  25 mm       1¼ hr        40 min          25 min