The long time standard for different baseweight classes of backpacking are:
Lightweight is under 20lbs.
Ultra-light is under 10lbs.
Super Ultra-light is under 5lbs.
Going under 20lbs is easy. Going under 10lbs isn't difficult but usually takes $. Going under 5lbs is beyond most peoples ability, especially for a thru-hike. A few have done it, but most people wouldn't want to.