The same way anyone would come up with a time estimate if asked how long it would take to mow the lawn with a pair of scissors or lawnmower -- experience and extrapolation.
A closer analogy would be a comparison between different models of lawnmower or between a lawnmower and a different kind of mechanical grass cutting device. In which case, it seems you would need to log time to see if you've saved time using one over the other.