To me, generally speaking:
1st year = mulligan.
2nd year, I expect to see significant progress. If I see regression, he is on a very, very short leash.
3rd year = last chance to show something. It makes absolutely no sense to wait any longer than this, hoping that sooner or later he’ll pan out. A complete disaster like London 2015 is grounds for immediate dismissal.
I think this is more than fair.