It looks like there is not preclearance from Quebec to US. So I assume we would have to go through customs at first stop in U.S. Any thoughts on how much time to add to connection to get through customs?

What is your itinerary?

It looks like flight options have 3-5 hour layovers so seems like that should be enough. Thanks anyway

All airports/points of entry are not created equal. A lot depends on <i>where</i> you are landing. (Immigration, collecting luggage, rechecking it, and changing terminals can take a lot longer at some than others)

Very very roughly, it averages for me about an hour from arrival plane door opens to go through immigration, customs, transfer to new terminal (if necessary) and reclear security. Of course, it could be 45 minutes or it could be 90. For me, at most airport, 90 minutes is minimum I'd book a connection; 2 hours is preferred.