What if I finish my 24 sessions in less than 6 months - can I advance in the challenge to the next 6-month milestone?
The challenge is designed specifically in 6-month increments of time, so you would not be able to have sessions beyond the 24 within that 6-month period count towards the next 6-month period; you can take extra sessions but only sessions in the next 6-month period of time will count towards the challenge.