In Dos when is it too late to shout "Dos"?
From the way I am reading the rules you can call dos on another player up until the point that the round ends. As soon as a player plays their last card it is to late to call dos as that would cause the player to get more points.

Offical Rules

Rules

Rules for calling dos


  Here is where Dos and Uno are similar. Once you have exactly TWO cards in your hand, remember to yell out “Dos". If you forget to shout “Dos" and are caught by another player (who says “Dos" before you do), you must draw 2 cards from the Draw pile as a penalty, but take note that you only draw the two cards at the END of your turn (when you are just about to end it).


End of round rules


  The first player who discards all his/her cards is the winner of the round. If the player earned any Double Color Match Bonuses during his/her turn, then all the other players should draw cards as well, before tallying up the points. The winner of the round gets to score points from all the cards in other player’s hands and become the dealer for the next round.