Here is some background information on marijuana. Keep in mind things like frequency of use, hydration, quantity, and type can all affect detection windows. These are just general guidelines that we typically hear toxicologists testify to.
Cutoff - 15ng/ml
Light use - 1-3 days
Moderate use - 3-5 days
Heavy use - 10+ days
Oral ingestion usually 1-5 days
1/2 life - 18-24 hours
Urinalysis shows use and not impairment
Greater than 150ng/ml generally indicates a recent use
50-100ng/ml is generally a low amount
Less than 50ng/ml raises the real possibility of a passive or innocent ingestion defense.