"I havent gone to a concert for a long time" or "I havent been to a concert in a long time," would be correct.

"I haven't been going to a concert for a long time" is technically correct, but it really doesn't make much sense. It's not a question a grammar; just a matter of what you are really saying. I think the big issue is that "a" means one concert. Maybe that's why your sentence sounds off when you try to put it in a continuous tense. However, if you say, "I haven't been going to concerts for long", it means you have a very new hobby. I agree with Paul's suggestions. Keep your message clear! Here are a couple of other options. "I haven't been going to concerts much these days." (This means you used to go often.) "I'd been avoiding concerts for a long time." (Yes, you use past perfect here.) "I don't usually go to concerts."