This question seems to be pulled straight from the classic 'Fermi estimate' [1]. However I have to say these are actually very useful in quickly assessing options given limited information. I remember doing something like this when discussing a claim about total information produced in history that another person felt was outrageous, I quickly threw together an estimate to show this wasn't outrageous and a physicist friend of mine pointed out the proper term for this. I can certainly see where the ability to do this on the spot would aid a product manager (and in this case I would say the ability to perform this estimate is independent of ever having heard the term 'fermi estimate')

1. http://en.wikipedia.org/wiki/Fermi_problem