More important than the class, is your own feedback. About 12 minutes in and you should be dialed in and running well. At 15-18 plus your attention may wander -thats the point to come in, I think sessions were 20 mins last year
Yes, The half hour trials always seemed to have a "dead spot" where concentration wandered, I'm glad the hour trials were long gone when I started VOC HSTs.
H