I haven't spent more than 15 hours on a song, but then again I haven't really done any big projects. And I mostly stick to short songs. But I can definitely see a 5 minute song taking more than 15 hours to complete. I'm very picky so I'm sure I could save a lot of time by~ being less picky. I also tend to listen to the song way too many times while making it.
When I have a good flow, composing is very quick but when I'm stuck it's like "wat do" so it really depends.
Also, thanks for this thread, seeing as I was curious about this subject too.