I would recommend using a version control system (eg. git, mercurial, bitkeeper, NOT svn/cvs
) while working on your picture (and saving as a new version after each change, each move of a pencil/brush) this way when you see it took you more than 5 minutes, you can always revert to the last version qualifying to these time conditions. This also allows you to work in a less linear manner, for example making two different versions of background, then merging them with foreground to see which one looks better...
on the other hand you would loose ~80% of your time dealing with version control system, and even more wasting time on branches you do not use in the end...
which basically leaves you with approximately 5 seconds to draw the picture...
that might be disqualified because you use advanced tools...
ok, this would be a bad idea after all xD
<this was a poor quality geeky joke>