NAC is only needed if you are putting things into your body that are hard on the liver. Such items would be any methylated drug (oral or injected), tren, and alcohol. If you are not using these (and maybe some others I did not immediately think of), then you should not need any NAC at all. However, using 600mg a day (usually one pill) will help your liver out even if you do not use any liver taxing things.
So my recommendation is to use 600mg a day if you are living a normal life and not doing anything harsh to the liver and doubling it if you are using liver toxic substances.