It differs greatly.
My European Borders map: http://warlight.net/SinglePlayer?PreviewMap=15479
took over a month of regular developement, from start to finish. (More if you include testing)
On the otherhand my Africa map : http://warlight.net/SinglePlayer.aspx?PreviewMap=19662
took under 24 hours to make. It is still in development, but not for long.
It really depends how skilled you are with Inkscape, my first map "South America and the Caribbean" took ages as I got used to the software, but my most recent map (with more territories) took 1/4 of the time.
It takes a long time, but if it is a good map at the end of it, that you'll enjoy playing on (And others will too) it is worth it.