How long does it take to do a portage?

To figure it precisely, I check my watch when I step out of the canoe upon arrival at the portage.  I check my watch again when I paddle away from the other end.  

End Time - Start Time = Elapsed Time  ;)  It's over when it's over and not before.

Seriously, though, I just don't give it that much thought, especially on portages I've never travelled.  There's too many potential variables.

Mad_Mat's system seems like a good rule of thumb to me if you're goal is just to make sure you aren't planning your days too aggressively.