I used to work at a drug testing facility. Our typical test urine and saliva took 3 major steps.

1) Negative Screening (6hrs) - Figuring out what compounds show up initially. Special dyes were use to identify potential compounds. Not conclusive but narrows down the next steps

2) Extraction (10 - 18 hrs) - Separate the target compounds from the rest of compounds/proteins in the sample fluid.

3) Postive Certification via GCMS (2hrs - 16hrs) - The extracted compounds were ionized (broken into predictable pieces) then sent through the GCMS to where a signal signature determined.

Each step included equipment expensive materials and equipment. The last step is was the most expensive and error prone. The GCMS machines need maintenance and calibration. They cost anywhere from $30,000 to $70,000.

Sometimes we lost samples because the machine would lose it's calibration and wouldn't be enough to start another set. We took in samples on a national scale, but I can see how a smaller non-commercial lab accumulate a large backlog.

This was like 10 years ago but I don't see the technology changing that much.

Yes. I had a relative die of morphine OD (needles! were still there next to him). It took months for confirmation from the medical examiner.

Real life is not a CSI episode, also labs have a limited processing ability

But maybe tests are done in stages? Like, if nothing obvious shows up on first tests then some other ones are done?

Not excessively (it's not excessively long, yes it's normal). Toxicology is usually at least 8 weeks unless we're talking about someone famous enough to have it rushed.

It depends on the lab's backlog. When I worked as an evidence technician, it would sometimes be six months or more between dropping off the samples and getting the results. I had one murder case that took nearly two years to get back, longer than my time in that position.

I wouldn't doubt it. I imagine it's due to a backlog and lack of funding.

Each test runs quickly, but there is likely a huge backup.

It's sadly pretty typical.