You can play the Trump Card anytime after the point, you have deduced the cards your opponent has left, and you KNOW by playing it, you will win the game.
You can play it later, than sooner, if you want to see your opponent twist in the wind a little. But if they are good card players, they will know they are beat.
I suppose they could turn the table over and cause chaos, but then they still lose. You might have to chase them down to get your winnings.