End of summer camp?
End of fall camp?
After the first few games?
After Zach returned from his injury?
I am a fan of sticking with guys through the bumps and bruises as long as they're progressing. However, if you see a guy that isn't progressing AND you have a capable backup who has proven effective...I think that you let that "fair shake" roll out as long as possible.
Competition is a good thing.