It shouldn't be more than 10 or 15 minutes although I don't speak from having experience mid-week/mid-day.  I just can't imagine it being heavy during the time you are inquiring about.

there is no average wait time. Sometimes you think it's going to be very busy and there's not much of a line. At other times when it's normally slow, there's a long line.

I've asked the customs guys and they say the same thing, you just never know when it's going to be busy or slow...

The wait stays short until the discos close.  Even then it's not bad.  Be careful if you drink and drive--the cops on both sides of the border keep an eye on motorists late at night.