How long before thc leaves the system?

That depends....: Single use marijuana might be detected in urine for 1 to 7 days (or more) and regular marijuana can be detected in urine for 7 to 100 days (longer periods have been reported though).

Weeks: Thc, the active drug in marijuana, is fat-soluble, which means it lingers in the body a long time — especially in trace amounts that can be detected on drug-testing. Depending on the type of drug test, it can be detected for many weeks. There's also no "antidote" to sober up from thc fast, if that's what you're asking. Best not to put it in your system to start with, if you need it to be out.