The number of people in line really depends on the time of day. I used the baggage check-in at POR in June. I got ther about 7:30, with dd. (keep in mind that everyone in your party has to be there...it's just like checking in at the airport). There was one group ahead of us. When it was our turn it took about 15 minutes to get checked in, our bags tagged and onto the cart.
But, when we went back through the lobby area about an hour later, there had to be 50 people in line.
At POP, in August, we got in the line about 8ish and it was about 15 minutes before it was our turn. I would leave at least 30 minutes, just to be on the safe side.
And of course, you must be flying on one of the 'participating' airlines to do this.