The first several topics of BC are the same as the AB curriculum. As my daughters’ school requires AB before BC, the teacher significantly accelerates them, covering them in a matter of a few weeks, iirc.
When I was in school, we had the option of AB or BC (I don’t remember whether you could even do them sequentially) so the AB topic were covered at a normal pace. I don’t recall the details, but suspect they should take approx. 1/2 the class.
College Board’s course definitions actually show eight of BC’s ten topics to be the same eight as AB.