How come sometimes they leave a cast on for 6 weeks and other times for 8 weeks?

All depends where the break is and how stable the bones are without being compromised by osteo and age.

To add to what Tick said, it also depends on the kind of break. Was it a compound break? A greenstick fracture? Etc. Also, what bone was involve? Ulnar? Femoral?

Too many variables to answer this completely.