The Collaborative International Dictionary of English v.0.48:

man hour \man hour\, man-hour \man-hour\n.
   The quantity of work which one person can perform in one
   hour; -- often an estimate made for the purpose of deciding
   whether to undertake a project, and sometimes used in
   accounting; as, it will take a hundred man-hours to write the
   program.
   [PJC]