Published studies on drug retention times are conducted on individuals that are not chronic users of drugs, and have received a single dose of a particular drug.  The times tend to be longer in individuals that are heavy chronic users.

The amount of time a drug remains in the system depends on the character of the drug, amount of drug taken, the frequency of use, the person's weight, any diseases present and the individuals metabolism.

Another factor is the concentration of the urine specimen.  Specimens collected after a person has consumed a large quantity of liquids are likely to have a more dilute specimen, and would therefore have a lower drug concentration than specimens collected first thing in the morning.