They will just know that you used.. They won't know when.There would be no way to determine when exactly you used. Even with testing the levels, they wouldn't be able to tell that.
It can but it is expensive. More often they will just test for the presence of a drug. They will then know approximately when you used the drug depending on the type of test eg blood or urine.
Can a hair folicle test tell when you last used opiates? Or does it just tell you you used in the last 90 days? Does it tell levels of use?
Yes, approximately. They can measure the amount of nanoparticlesand based on the amount can estimate your last use.