I do not have a specific date but evidence points to 1979 based on this quote, the link I will provide as well:
Kevin's players urged him to formalize his rules, write the fantasy game and sell it. He did just that, and pitched the idea to every role-playing game company of the day. They all turned him down. Actually, Judges Guild made Kevin an offer that he found much too low to accept. Nonetheless, the owner of Judges Guild liked him and his artwork, and hired him as a freelance illustrator.
Quote SourceI assume, since Kevin did work for Judges Guild for a brief period (1979-1980), the above refers to a time just (maybe a matter of a few months or even weeks) before he submitted any illustrations.
Those who can, don't. Those who should not, do.