I don't really know. I thought there would be difference in concentration.That concentration depends on the person: A person with more fat will show a higher concentration for a longer amount of time. If you don't smoke often, you might find that you get it out of your system rather fast. In short, tests aren't all that helpful with pot, and you won't realistically be able to tell if they smoked last night after work or last saturday during off time.
> Does it matter if it is a "heavy" drug (say, mushrooms?)?
Mushrooms are not a hard drug. Fentanyl/heroine and crack are.
Mushrooms and LSD are classified that way by many folks, though - including the US government. Fentanyl isn't instantly addictive - if it were, they wouldn't use it in hospitals. People do heroine occasionally without being addicted, too (most folks don't shoot up). Same with cocaine. I don't know as much about crack, but there was a time when folks did meth on a more recreational basis instead of all the time (I've never personally done it, no desire).
And that's the issue. "Hard" depends on some combination of use, effect, and what frequent use does. Regardless, plenty of people do "hard" drugs occasionally and you'll never know. And if it is better to you that someone do some cocaine - generally accepted as a hard, addictive drug (through propaganda) - than be drunk all weekend, in some ways, you already know this. A drug test isn't gonna help most of these, though.