Hot water tank running out?

Tried searching online and could not find an answer. We live in a very old house and of course in the summer it's no problem but in the winter our hot water tank sometimes runs out before we are quite ready to exit the shower. I am not sure how big the tank is, but at about 15-18 minutes you can no longer... show more Tried searching online and could not find an answer. We live in a very old house and of course in the summer it's no problem but in the winter our hot water tank sometimes runs out before we are quite ready to exit the shower. I am not sure how big the tank is, but at about 15-18 minutes you can no longer increase the temperature of the water and it slowly gets cold. My question is, does having the water at a very high temperature make the tank run out faster or is it just the amount of water the faucet puts out?
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