Can someone please explain drug half life to me?

I thought i knew but i am getting confused. Let's say that Hydrocodone has a half life of 4 hours. If someone was to take 5mg would this mean that in 4 hours 2.5mg has been metabolized (active and effective) and in 8 hours 3.75mg has been effective? So after 8 hours there is still 1.25mg to metabolize or whatever? I read something about multiplying the half life by 5 to determine the length of effects. And how can you tell if a drug is fast acting? I thought that fast acting drugs would have shorter half life but i was wrong. Xanax has like a 10 hour half life and it is considered fast acting? What the hell? And oxycodone is 4 hours... Can someone explain this to me? Thank you.