AFAIK, Auto Trial is good for automatically setting regular CPAP pressure or to simply try Auto and see if the results are better than in CPAP mode. Winterfrost can already tell his 90% pressure and use it as a guide to set the min and max.
As for the original question: After starting the treatment, the improvement for me was gradual yet, right away, I noticed that I was waking up much more rested than before treatment. 11+ years ago, I didn't look at AHI nor any other data. I simply felt better and that how I knew that the treatment was working. Now, I know that it's still working because; once in a blue moon, I have to sleep without my machine and than, in the morning, I'm feeling worst than before falling asleep.
I reserve the right to change my mind. Why? Because tomorrow I may know better.