There are several factors that must be considered.
As stated before, each drug has different windows of detection.
It also depends on what model of drug testing is being considered.
For example, in regulated testing (such as DOT testing) there are specific thresholds set by government regulation which play an important role in "windows of detection".
The biggest impact that a diuretic will have is in "peri threshold" concentrations of drugs. For example, if the cut off for reporting is 300ng/dL, a diuretic could push that concentration down to 299ng/dL, which would be reported as negative. Scientifically, there is no difference between 299 and 301 except one gets called negative and the other is called positive.
Virtually all of the products available on the net that are reported to enhance excretion actually work by volume loading (typically there is a paragraph somewhere that recommends taking the product with "a minimum of 1 or 2 litres of water" which is a clever way of saying "volume load" to dilute the sampel).
Final comment relative to diuretics. Virtually all tests assess specific gravity and/or urinary creatinine as indirect measures of hydration. Many laboratory tests include tests for diuretics since they are commonly used to cheat the UDT process.
Good luck,
nitrous