For me, the answer is months. 
The one question every artist is bound to be asked is "How long did it take to paint this?" If the answer is 45 minutes, the asker will likely start considering starting up his long-dreamed-of art career.  After all, $1200 in 45 minutes means $1600 an hour, and that's $64,000 for a 40 hour week!   A friend of mine, Perry Austin, (www.perryaustinfineart.com because I don't have an 8 year old handy at the moment to make a cool hyperlink) answers with his current age, which makes a lot of sense -although many askers don't quite get it.  My favorite all time answer comes from  Ted Goerschner (http://tedgoerschner.artmystyle.com/ ): seven generations of selective breeding.