Within eight hours of taking an oral cephalexin dosage, 90 percent of it will be gone of your system. Cephalexin is excreted by the body through the urine. For patients with impaired renal function, this procedure may take longer. If you are taking cephalexin before or after a surgical procedure, the amount of time you should wait before taking another dose is three days for the chest and two weeks for the abdomen or pelvis. Cephalexin remains in your system for up to five days. However, if you don't take the last dose by the end of that time, most of the drug will be flushed out of your body within a few more days. The half-life of cephalexin is about two hours. This means that it takes twice as long to get rid of it in your system. The amount of time it takes for your blood concentration of cephalexin to drop to half its original value depends on how many doses you take over the course of your treatment. For example, if you take two doses per day, the concentration will be low enough for only half as long as if you took four doses per day. You should not take cephalexin after the expiration date printed on the packaging. Follow all instructions on your prescription medication container regarding storage, disposal, and dosing guidelines.