Question: how long does it take for hot water to run out?
Answer: Say your electric heater is 30 gallons. If the previous shower lasted 15 minutes, you're looking at about four minutes of hot water remaining. If you want to loll in your own 15-minute shower, you'll need to wait 47 minutes before stepping in. (Gas units heat faster, so multiply by 1.9 instead of 4.3.)