The twenty year timeframe misses the differences between succeeding and failing. You may not be able to tell if something is success until it has completed every stage, but you can detect failures much much earlier. If we used a space flight as an analogy you cannot say it has succeeded until the very end, but there are lots of points - launch, first stage separation, reaching orbit and so on.

And that is true of most things: you can detect something had failed/not-yet-failed many times and much earlier than the eventual success.

So you can see if Brexit is on course for success at least monthly over that 20 year period.