If I smoked 3 grams of marijuana in one night, how long would it take to leave my system? I was a chronic user up until a month ago.

A long time. The half-life of THC, which is the active ingredient in marijuana is 30 days. In other words, 30 days after you completely stop using marijuana, 1/2 of it is still in your system. 60 days later 1/4 is still in your system. 90 days later 1/8 (12.5%) is still in your system. THC is fat soluble, so it gets in all your fat cells & slowly diffuses out. Your brain is 60% fat. .
Leave system or? It may take several weeks to fully leave your system, as some of the chemicals in marijuana are fat-soluble and can stay in your body a long time. If what you're really asking is how long it could be detected on a drug test, it might be 1-2 weeks before a urine test would be negative. However, after chronic use it can take up to 3 months for urine to test negative. See http://tinyurl.com/c5nsv2j.