I am going to have a drug test in a week or so and I am addicted to painkillers. I need to know how long they stay in my system for, so I now how long I have to stop, if I can stop.

Each one is different. Search for "physician prescribng information <drug name or component>" Find the "half-life" in the information. Multiply the half-life by 5.

This does not apply for a hair test. Urine may be faster.

As noted above much depends on the specific drug. Also on the type test being used. Many pain killers (synthetic and semi-synthetic opioids) aren't detected in standard 5 panel tests.