According to Dr. James Okamoto on Health Tap, it takes 5.5 half-lives of a drug to leave your system: Based on this fact, 5.5 times the half-life of Flexeril, which is 18 hours, equals 99 hours. Flexeril's half-life reveals how long it takes half the drug to leave the body.
Flexeril is used to reduce muscle spasms and is a skeletal muscle relaxer, according to Drugs.com. Flexeril is the brand name of the generic drug cyclobenzaprine. It is available by prescription only; and, it is closely related to the antidepressants imipramine and amitriptyline. Drugs.com states that it should only be used for two to three weeks.