1x a week if you are starting to stretch. I think the point is if it's to begin stretching and to minimize any downtime from being overzealous in stretching. Many people start a stretching program and go way overboard and end up needing days off to recover they get so stiff.
So it's just conservative and progressive and ramps up from there. This would make sense considering where Coach Sommer propagates is slow, patient progression.