That happened to me in one of my first games reffing. A coach asked the time, i said 5 mins left. There was an injury where i stopped the watch for about 3 1/2 minutes. Restarted the game, played the extra time and the other team equalised. He was jumping up and down on the sideline saying that i'd played too much time. After the game, i reminded him of the injury to one of his players and he still wouldn't accept it, so from then on i've been as vague as possible - 5 mins plus injury time would be my answer now (regardless of if there has been any injuries or not).