This comes up on FT fairly often; do a search. For a ballpark estimate, the northbound wait is typically 30 to 60 minutes; southbound on Sunday afternoon/evening will likely be longer, maybe much longer; my record is three hours.

The "truck crossing" mcrt refers to is one mile east of the main Peace Arch crossing that I-5 northbound funnels into. You want to exit I-5 at the last Blaine exit before the Peace Arch and follow the signs. Southbound, same trick: get off BC Highway 99 at the last exit before the border, drive east to the first traffic light, and turn right/south. The southbound line is almost always shorter than at the Peace Arch IME. Also, there is a good duty-free shop in BC on the right side of the road just before the truck-crossing checkpoint. You can get on-the-spot GST refunds in there if you've bought anything substantial in Canada.