For the most part I think they wait until their mass bannings. If you're duping and shit like that, they'll probably ban you individually, but for maphack, people rarely get banned except for in the mass bannings.

Also, I've heard that since the last patch, the Warden has been turned off. I don't know if that's enough for me to think that it's safe to hack or anything, though. They could just be updating it, or giving people a false sense of security.