I'm sick of all of the bullshit about how long it takes to burn a body based on eyewitness/expert testimony from random nobodies who might be lying, doing the math on this topic shouldn't be difficult.
The human body is a certain percentage water, to eliminate that mass, the water portion needs to increase from room temp of 20ºC to 100ºC. It is very easy to calculate what amount of energy is required to do that and that eliminates most of the mass.
There is also a remaining mass after the cremation, whatever percentage that is added to the water percentage leaves on a small fraction left to calculate.
That energy input to eliminate the remaining non-ash, non-bone, non-water portion is easy to estimate using the same calculation for the water, but with a top temperature of whatever the oven is set to.
There you go, very easy, basic science and math. No more need for "my cousin's friend was an undertaker and he says it takes 5 hours".