Yeah thats a funny one too, noticed that sometimes its quicker than others but not sure why this is? Or if it correlates to what position you get?
Pretty certain I got over an hour on the 8.17 one as it was only recent - think it was Southampton Local General, but I don't think it effects your queue time. I think the ticket office must just open the queue at slightly different times around the 8.15 mark.
I've had refreshes at 8.16 in the past and had 5 minute queues