I consulted the timestamps of the photos from when I went through the line. The first time it said 150 wait, and it took about 90min, with one 15 min breakdown while we were in the greenhouse. The photo of my parents at the entrance to the Castle part was :52, the picture I took of the sorting hat, which is after the test seat, and right before load was :06. The second group of photos: right at entrance :35, and I have a pic from the Common room at :49.
So about 15-20 min from when you enter the castle. And most of that wait is the line stopping to take photos.