Create a variable with the amount of time you want it to last.
Create a function that every tick will subtract a value from this variable.
I suggest you to subtract to this variable the value of " 1*dt "
dt means Delta Time and it will make the game subtract 1 every second independent of the player's computer frame rate.
Now create a condition that when the variable value hits 0 will call the function "go to layout" if you want to change layouts, "set group active" if you want an specifically part of your code to be executed, or whatever other event that you want to happen as soon as the count down reaches 0.
If you want the count down to appear on the screen use a text object and set to every tick to chance it text to the value f the variable.