I was told by a test administrator that generally they are undetectable after 3 days. She said the same goes for benzodiazapines (valium, ativan, xanax, klonpin, etc). Based on that I would venture to say that the most common rx are out of the system fairly quickly dependent on weight. From what I understand the stuff stores up in the fat cells. There is a weight break, maybe around 160-175 where instead of 3 days it takes 4...but I don't really remember. Since I have prescriptions for what I take and work my hardest to not abuse, I don't really worry about it anymore. But it used to really freak me out. I remember when I passed my first screen on my own. It was amazing.
minumum of 36 hours..........
Half Life of Hydrocodone is 4 hours (if I recall correctly).
There is some scientific method for figuring how long it shows in your system (Quantity taken / Half Life + ABCD) - I don't even want to pretend to know how it's figured out. I do recall reading somewhere that it can show in drug tests for a week to two. If you have an old script, maybe you can show that as the reason for the presence?