Nearly anywhere in the game!
Whenever you have a waiting time, say for production of resources, for building/upgrading, for sending out a scout, for producing troops in the barracks, for crafting in the Magic Academy and all the other places I just forgot.
If you click on the building in question you will see your time progress, just behing the progress bar you will see a button (Not sure if all versions of the game always have an hourglass on it), anyway click the button and you get the option to select any of your timeboosts to shorten the waiting time.