I like using the headphones during practice. How much drainage is there to the batteries, if any?

100% the same as when using a 1/4" cable. If something is plugged in the 1/4" output the battery is draining. Inserting a connector in the jack is literally the power switch for the bass.

Makes sense. ok. I need to get a rechargeable 9 v battery and charger machine! lol.

I may have a solution or at least an improvement for the play time with the 9volt batteries. The picture below is an 850 mAh lithium 9v RECHARGEABLE battery set of four batteries.
The regular batteries bought in stores are typically only 600mAh, which we know only gives us the 20-hour play time. I haven't recorded or waited for the 850mAh to die but played for several hours. When I later tested the batteries with my voltameter the power rating was still in the green(good). So, I know these batteries will last much longer than 20 hours. Possibly 30+ hours. Mainly, this route will save money from now on.