Though some measurable amount of chemical remains in the body for up to a month. Impairment is a temporary thing. As with alcohol quantity determines length of effect.
Quality can determine length of effect also.
Actually, when it comes to cannabis, quality affects the intensity and type of high rather than how long it keeps you impaired. Most high quality cannabis will give you an intense high after only 1 or 2 hits, but it will wear off after around 1 to 2 hours if you don't keep smoking. Eating sugar helps to level a person out from a high as well. And..........the reason that it is able to be detected for so long after the intoxicating effects have worn off is because THC bonds to fat molecules, and because of that tends to stay at detectable (not intoxicating) levels for anywhere from a week to 30 days (depending on body fat percentage).
Why are young Republicans in favor of legalization and the older ones aren't? Because the older ones still worship at the altar of Reagan, and in doing so want to continue his "just say no" policy towards marijuana.
Interestingly enough, research has shown cannabis to be many times less dangerous than alcohol or cigarettes.
No High Risk Marijuana May be Less Harmful Than Alcohol Tobacco - NBC News
I think that marijuana needs to be removed from Schedule I substance classification, because if a substance is Schedule I, it is harmful, highly addictive, and holds zero medical benefit.
Marijuana has been PROVEN to have a medical benefit for a variety of ailments, from glaucoma to Alzheimer's, to seizures, to a whole bunch of other diseases. It has also been PROVEN to be physically non addictive (yeah.......there are those who can become mentally dependent on it, but the same can be said for shopping, gambling, co-dependency, etc.), and has now been shown to be less harmful than alcohol.
Personally? I say legalize it. Matter of fact, if they did that, I'd give up beer for cannabis.