"Banging" means snorting or smoking cocaine. He probably sniffed a ton of coke, I can't see Charlie being a crackhead. Cocaine, as we all know, is very common in Hollywood.
He's been on the stuff for years.
Over time, he probably needed to do more to get high (obviously). I'd say doing seven grams at a time is insane, but I think he meant he did seven grams in a sitting, or a night, and it was nothing to him.
How's that for translation?