There is no official rule for when the entire deck is used and the game hasn’t finished. We suggest for your house rule that you call “final round” when there are still enough cards left in the deck for each player to have one last draw. With 6 players this would mean calling the final round when there are 6 cards left in the deck. You may not use all the cards if some players choose to pick from the discard pile, but this would give each player one last turn and be a fair way to end the game.