The fact that you are concerned with repercussions for stopping a 'bible study', should in itself indicate to you that something is wrong.
I highlight 'bible study' as you may not have initially recognized, you were likely not studying the bible but were studying Watchtower publications which sometimes refer to New World Translation (JW bible) passages, often out of context.
It is your right to stop any activity you do not wish to do. While you express concern over wasting their time, your conductor counted her time spent with you. JWs knock on doors and conduct studies as a means of counting time in preaching, to measure how spiritual they are.
You are not a baptised JW so there is no penalty to stop the study as you will return to be a person of the world.