From what i understand you need to have had a test within 90 days of the start of employment.
If you have an incident/accident you are supposed to be tested within 2 hours of the event, which is great if you are out in the middle of no where. What your supposed to do is carry a test kit and test yourself!
Interesting note is you dont need to be tested to get your SPL, first place to start i would of thought of.