'Ass to grass' — the deepest possible squat. In practice what counts is hips below knee height without the pelvis tucking.
The squat-depth debate is old and usually badly framed. Deep squats aren't inherently bad for knees — on the contrary, a larger range builds more muscle and trains the knee in positions sport demands anyway. Your limit is individual, set by hip anatomy and ankle mobility rather than willpower. The decisive marker is the butt wink: if your pelvis tucks under at the bottom and the lower back rounds, you're too deep under load. The right response is to work on mobility (usually the ankle) and stop for now where your back stays neutral — not to force depth with weight on your back.