I've think that the broadest trend is basically how skilled/prestigious the job is, and how difficult it is to replace people.

• A minimum wage retail worker? Tested the moment they're hired, or if they act suspicious, at random, or if the company wants an excuse to get rid of them. Almost certainly mandatory termination if anything comes back positive. Not for every company, but probably for a lot of them.

• Entry level office work, salesmen, tradespersons, unionized industry professionals, teachers: Probably tested at hiring, maybe with random tests, or after poor performance, and almost certainly after an accident. No-tolerance, mandatory termination policies in some fields, alternatives in others (especially those with unions).

• A lawyers, media workers, upper management, scientist, programmer, other highly paid professions: Probably never tested unless there's a serious performance problem, quite likely to have alternatives to firing. My sister is a clinical psychologist working for the government, for example, and has never been drug tested, but they reserve the right to do so. But if they do and she comes back positive, nothing would happen unless she failed another test given thirty days (by which time anything would be out of her system).

Obviously this varies a great deal by company, as well as profession. People who do dangerous work or have to deal with public safety are tested more rigorously than office workers, for example. And the consequences for getting tested positive for weed may be different for meth, heroin, etc, once you get above the level of no-tolerance mandatory termination industries.