It's quite simple maths. It takes about 4000 joules to raise one litre by one degree. So 10 litres from 15-100 degrees takes 4000x10x85 joules - 3,400,000 joules. A 3Kw hob of any sort will deliver 3000 joules per second so divide by 3000 to give 1333 seconds. 22 minutes. Allowing a bit for...