How many hours per week would the class probably be?
The class itself is 1.5 h/week.
Are you asking about the homework? It is difficult to say. May be 5-7 h or sometimes 10 h and up (per chapter, which is roughly the same as the week of class, 12 chapters = 16 weeks). It could depend on the topic.
You could get the books now and try. The class starts Oct 17th, and may be cancelled before the 3rd class (which would be in 2 weeks), for full refund. So no harm in registering.
The next class after that starts Feb 9th.
If somebody can help your DD locally, you might not need the class.
ETA: DS had EPGY Math K-7 before AOPS prealgebra (by book).