This is explained in the global forum rules. You lose one point (10%) after 30 days of good behaviour (in other words, 30 days without getting a single warning) starting from the moment the warning was issued.
Anywho, at the time of this post I have a 0% warn level (but a total of 10% lifetime which is now long gone for derailing a topic back when I was a member) and 1144 reputation.
EDIT: 1145. Why must you do this to me, NinjaGamer598? D:
EDIT2: I'm not going to keep updating this post, by the way.