Hi Nick,
The battery powered CBEAST when pressing hard for heavy correction will last about 10 to 15 minutes. If polishing LIGHTLY - the battery will last up to 20 and 25 minutes.
I LOVE the cordless rotary, but I rarely use the cordless CBEAST. But I have my own reasons and the biggest reason I don't use the CBEAST is because most of the cars I personally detail, and most of the cars I bring in for our classes are in REALLY BAD CONDITION. Thus, I like the corded BEAST or the corded Supa BEAST, and I simply deal with the extension cord.
I much prefer to go faster than go slower.
And keep in mind, with a cordless rotary, the battery/motor only had to spin a buffing pad in a single direction. With a gear-driven orbital, the battery/motor had to spin the pad in two directions, rotation and oscillation. This extra power demand drains the battery faster.
And as far as the 15mm free spinning, random orbital polisher, aka the FINISHER, because it's NOT gear-driven, the buffing pad can simply stall out when buffing hard or buffing curved panels and there's less energy demand thus longer run-time.
I wish all the cars I work on were in like-new condition as then I would be cordless for every car.
Not sure if the above helps, but that's my take.
Mike