Point this out to a JW, and they will probably tell you that Jehovah is patient and that one thousand years is like a day to him, and a day is like one thousand years.
Then, ask them if "very soon now" means 50 years. Or one thousand. Or anything at all.
Ask them: if the end might not be for another thousand years, why the urgency?
You probably will not get them to understand the problem with saying "very soon now" for a hundred years. At least not right away. Maybe they will understand it... very soon now.