Iv e had a lot of them and mine were never more than 30 minutes including a rest period. The only thing i would add to what tranquility said is that you blow out as hard and as fast as you can, they will explain it. Also you will have to do this at least 6 times total. Nothing to be concerned with. They are looking to see how much your fev1 (expulsion of breath the first second) increases with the use of a bronchodilator.

Now a methylcholine challenge thats another story.