I've had the policy work after ~150 years in BYZ game, while it took a mere 5 in my JAP game. It's just like any other event, the MttH is just a rough idea of how long on average it should take, not a countdown. Since there are no modifiers to the MttH, the timing should be somewhat more consistent than most events though. Also, moving your focus does not affect the modifier or event, so it's OK to move it after making the decision.