Time to complete 2 interviews?

I'm planning to do the Canadian interview in YYZ in Dec. Once I do that, how much time would I have to complete the 2nd (US) interview.

I'm not flying in Dec. so thinking to just go to airport to do the Canadian interview and then do the US one at a later date (when I'm actually travelling to the US). My understanding is that I cannot do both interviews without having an airline ticket.