Not very long at all, never see much of a line...

I use YVR at times as a substitute for SEA on flights to/from Europe and always find preclearance to be very quick for afternoon/evening connections. At times I miss my onward connect back into the SEA (because Canadian immigration lines) and have to overnight at the Fairmont in the airport and take the first flight the next morning, and rarely see any line then either. All the US carriers are lined up in a separate check-in that is for US bound flights only, just steps away from the Fairmont entrance. I am usually out of the hotel and through security and preclearance in 15 minutes max in the morning. It opens quite early, there is an Alaska flight that it almost hurts to have to get up for...