Well, I'm not Melody, but I think I can answer your question for you.

Firstly, the half life for valium is about 12 - 18 hours, meaning half of the drug will be out of your system in that time. But, the thing with half lives, if you remember from chemistry, then the process starts all over again for the remainder. Meaning half of what remains will be out in another 15 hours or so, and so on.

So, it takes a lot longer than two half lives to eliminate a drug from your system.

It takes about seven half lives to totally eliminate any drug from your system. So, doing the math and assuming an average of 15 hour half live, it would take about 4 1/3 days (105 hours) to eliminate the drug totally.

Now, keep in mind however, this does not mean that Valium is effective for 4 days. The level required for therapeutic dose of most drugs is generally at most one half life, and even shorter if you’ve used the drug for a while and developed a tolerance to it.