Far as I can tell, Colorado courts have already settled the basics of the issue, though I'm sure there will be some refinements. Basically all the cards are in the employer's hand and an employee can be fired for off-the-job use even if the person is not impaired at work. People who were required to undergo drug testing previous to legalization probably won't see any change and their hiring or employment will still be contingent on being drug free.

"Nothing in the law will "affect the ability of employers to have policies restricting the use of marijuana by employees," the amendment states.That includes getting high at work or even after hours, according to legal experts and judicial rulings.
Tests can't determine exactly when pot was ingested. Marijuana metabolites can remain in the human body for weeks, so employers don't know whether a positive test resulted from on- or off-duty use. Nor do they care, if they have a strict no-drug policy."

http://www.denverpost.com/ci_2310482...duty-marijuana