What other data do I have to go on? PBPs have only been out for like a year. I figure I'll use the new ones in patterns similar to the old ones. PBPs have a bigger battery, so if I get two years out of AirPods it's not unreasonable to expect the same or more out of PBPs.

Miniaturization carries a stiff penalty. I'd expect the size-class needs to be similar before one can make a meaningful comparison.