Probably a bit later, if you leave enough time for TBS officials to include budget initiatives to be included in the budget, so that when the budget gets debated in the House shortly thereafter, probably sometime in early April, you have main estimates that fully reflect budget proposals.
In the meantime, you have interim supply to keep the government going, but in April, committees and Parliament debate the mains, and the mains fully reflect budget initiatives. That's how, in a nutshell, I would see that.
That would require close collaboration between Finance and the Treasury Board Secretariat before the budget. Finance could not work in isolation on developing budget proposals, but my understanding is that Finance and TBS already have improved working relationships in the pre-budget period. They would probably need to bring that one step further.