I think the answer is it's both a function of supply and demand as well as advance purchase requirements.

Looking at YYZ-YVR, it seems like all of the Tango fares have minimum 5-day advance purchase and most are 10-18 days. Some Flex fares have 5-day or 10-day advance purchase.

So, if you book less than 5 days out, you're guaranteed at least a V fare. Of course, it's possible that V or other lower fare classes could be sold out earlier than that if people buy them up.

How much the price goes up as fares expire depends on what has been bought. The difference from the highest W fare (5-day advance purchase) to the lowest V fare is only $1. But lowest G fare (10-day) to lowest V is $62, and if V sells out quickly, the price goes up quickly in Q, H, U, M.

As for demand, I don't think one can generalise for all of domestic/TB, but there may be patterns to specific routes.