I hate it when my (non-artist) friends ask me to draw something for them

…because I never know how to politely tell them to either pay for the work or fuck off.

To them, drawing and painting is just a fun activity that artists do purely for their own enjoyment. Even if they draw for someone else; It’s just for the shits and giggles, right?
Actually: No.
Just today someone has asked me for a dinA4 (roughly 12x8.5inch) printable painting they can use as a cover for the song he wrote. -I will not revisit the whole conversation here, because thinking about it makes me want to pluck his eyes out with pencils.- Let’s just say that when I cautiously implied that such a painting would easily take more than 10 hours, he replied with ‘then just do it please’.
…
For the reason mentioned above, he probably isn’t aware how disrespectful and inconsiderate his request is, so I’ll write it off as pure, undiluted ignorance towards all things art and how much ‘more than 10 hours’ are, when you spend them sitting in front of a pc painting for someone else, while you really should be studying color theory instead.

Unfortunately, as bluntly as I can put such things as anon on the nets, I can’t get myself to say it to his face. I pretty much agreed to do it…though: ‘Paid works have priority so I’m not sure I can do it within less than 4 weeks. -or do you need it to a specific date?’ A lie. Maybe it’ll throw him off enough to request something smaller. Can’t get any worse.

Has this ever happened to you? If so; How did you tell the person to ‘pay or fuck off’? Advice would be very nice, since I’d really like to avoid such situations in the future :| 

-Anonymous