I've spent a while trying to understand the government's proposals, which, as others have said, are not super-clearly defined. Based on that, I would assume that there are going to be a number of years of additional planning, so it would be a minimum of 10 years to implementation and probably more like 15 years.
If you're looking at mechanisms to reduce cost, I would suggest that early land acquisition is one mechanism to substantially reduce cost if you know where the corridor is going to be. That, however, requires a lot of planning in advance.
I think a fellow panellist pointed out that it's a better idea to have the plans as well developed as possible from the beginning and then move forward quickly with land acquisition to reduce those land costs.