2-4 hours per minute, depending on the complexity of the song and how easily I can come up with and implement my ideas, and spread over several days or weeks depending on the length of the song and how much I want to do other things in that time.
To finish a map, you need to do two things:
1. Make yourself map even when you don't feel like you have a lot of motivation at first. As long as you aren't actually reluctant to map at the time, it shouldn't be too hard to get into it once you start.
2. Don't stop mapping too quickly once you've started. Reducing distractions can be useful. It can't always be helped, but that's life. Sometimes you can kind of make yourself get back into it, though, so try to do that when possible.