Soon, can't tell you exactly when, many buyers, particularly corporate and agency buyers, soon they will realize that buying a Intel based Mac will be a bad investment and they won't pull the trigger on a purchase.
EDIT: I left out the conclusion of my original post. The upshot of what I'm saying is that if the sale of Intel based Macs drop significantly, Apple will have no choice other than to speed up the transition, which means things could happen much faster than what your chart proposes might happen.