I hope you don't mind but I did a test of my own(trying to use the same settings too). As you can see, of a picture that small size and little complexity, going from ultra setting to a maxed user settings goes from 5mins to 1hr40mins. That is a huge leap! :)

However, if you look at sittingblue's example, at the best quality and 12 subrays looks almost as good as 25subrays. Now I am sorry, but I didn't have time to run a full test, but my same scene but at 12 subrays I let it render for 5 mins and the estimate was around 55 mins. I don't know how accurate that is, but at that same point the 25 subray estimate was well over an 1h15m.

Just thought I would share my timing with everybody. :)