Did the US preclearance this past Saturday on my way to Atlanta ex YYZ. My flight was at 8h50 and I arrived at Pearson at 08h05. When I saw the lines my heart skipped a beat as I thought I might not make it. As it turned out I had lots of time. Got my boarding pass after a short wait since one of the J class agents was tied up with a pax for as long as I was in line. Hint, if you are late, "try" to use the automated kiosks. The agents who handle baggage for these users also handle J paxs when free so you can always go up to them and say you were unsuccessful with the kiosk. Shareholder is correct about the line for connecting passengers which is way shorter than the main line which was longggg! My agent suggested I use it (formerly it was for J paxs but US Customs nixed this preferential treatment) which I did and was through in minutes. Total time from check in to clearing was less than 15 minutes which left me with 30 minutes to spare. So if last Saturday was any indication you should be fine getting there an hour ahead.