A human being can sustainable produce 50-150 watts of mechanical energy. That number seems pretty consistent across most sources I've found researching various energy topics. A gallon of gasoline has roughly 34 kWhrs of energy in it. So at the high end assuming a 30 percent efficiency in a gas engine, and a fit human say 34 x 0.3 /0.15 or 68 hours of labour for the labour of 1 gallon of gasoline. I don't like when they use the full energy content of gas and substitute it for labour as it does not take into account the efficiency of use. I did just amend my original numbers as I was using numbers for younger more athletic types. I was fascinated with human powered machines for a while. There was even a thread I used to follow where a recombinant bicycle was hooked up to a hydraulic pump to run tractor accessories. There are efficiencies to be gained though to cut that number down. Take an excavator for example. Having hand dug many 4 ft deep 25 in wide footing holes I can tell you they are awesome. They can easily replace my 24 hours of digging with 1 hour of work. Then it gets messy though. They get freighted in by truck, must use roads that are built with fuel, often coated in asphalt, and have a staggering embedded energy number. So all those numbers should count against their efficiency but I never see that part written down.