The Half-life of a drug

[DeLee]

Greetings.
I want to get a grip on this with a drugs half-life.
I understand that if a drug has a half-life of 6h then after six hours half of the drug is gone from your system,
but how many "half-lifes" must there go before a drug is TOTALLY out of your system?

I read somewhere that it was 4-5 times the half-life,
So if we take Ketamine for an example, the wiki page states that Ketamines half-life is 2.5–3 hours,
so by adding 5 with 3 you get 15 right, does that mean that the Ketamine is TOTALLY gone from your system in 15 hours?

Thanks.

 

[QUARE]

I've been told by sekio that it generally takes 7 half-life to be undetectable in your system.