The T2 immigration line up is always less than 15min if you get there when it opens (around 5:15am). I never have any trouble and I'm always waiting for the ML to open at 5:30am.

The REALLY bad times to avoid are betwen 6:30am and 8am (you might as well show up earlier and wait in the lounge) and 10am and 12pm. Of course, these are all weekday times. I've found that there is no rule during the weekends.

FatBoyYVR - if your luggage is checked to LGA, it will be moved from T1 to T2 and you will have to pick it up at T2 to take through Customs and then put it back on the belt again just before security. I'm not sure where the T2 pick up area is - I've just heard the FAs and GAs tell passengers what to do in this situation.