So, when you start the game, until you don't unleash the horde, the timer will not count. After you started the first wave, the timer will count, as well as during the Go Breaks. So after unleashing the horde you should hurry during the pauses or the Go Breaks to place the traps. Obviously, when you press Esc, the timer will stop counting. By the way there is an option in the Settings, under the Gameplay menu, where you can turn on a timer, to show you, how many time did you spend on that session so far. On the Orcs Must Die Wiki, you can see the time limit. I hope this helps. :) If you have any other question, i will try to answer for you.
