The subscription covers a whole academic year no matter when you decide to purchase.
Naturally schools want to choose when they roll out BECOME with their students.
The recommended, ideal time allocation to run the whole BECOME program in a school is 20 hours (although some schools get great results from a very lean 8 hour program).
Regardless of when (i.e. in which semester or terms) you decide to teach the program and how you pace it, you'll get value out of it.
Teachers value the learning outcomes but also praise the ongoing relationship benefits they see from running BECOME with their students.
If you're looking for maximum access time, get your subscription set up to start early in the school year: the price per student doesn't change if you buy later in the year.