As of April 1 the government intends to have the plan open for business. This means that projects will be identified, prioritized by municipalities, provinces, and the federal government. Obviously, smaller projects tend to proceed much faster than bigger projects because they're less complex, but as soon as projects are approved, the municipalities or the provinces can get on with the work. As soon as they have interim eligible expenses and the proof of those expenses are provided to the federal government, reimbursement can begin.
On March 25th, 2014. See this statement in context.