Frugality is founded on the principle that all riches have limits.

Care with money rests on one plain fact: whatever you have is finite, however large it looks just now. Spending as though the supply were endless is not generosity but a failure of arithmetic, and the reckoning arrives eventually for nearly everyone. Frugality is taking the limit seriously in advance rather than discovering it. It applies to time and energy exactly as it does to money.

Look at whichever resource you treat as though it will not run out. Put an honest number on it and see what that changes. Knowing the size of the pile is what makes deciding possible.