As deep as you can…while maintaining the proper form and posture.
There are certain things we look for in a squat. You want to keep your weight evenly distributed across your heel, the inside of your big toe, and the outsides of your foot. You want to drive your knees out and over, but in line with your feet. You want strong, but open hips, and a solid, braced core and midsection. You want to keep your chest upright and your back straight.
The squat is a very complex, compound movement, but once you’ve mastered all of these cues, start sitting as low as you can. Everyone’s squat is different because everyone’s body is different, and there is no one-size-fits-all approach to squat depth. Mobility restrictions in the ankles and hips prevent most American adults from having the capability to seat deep in a squat, bodyweight or loaded.
Unless you are powerlifting, training for strength,or have some other injury consideration, there is really no reason not to squat to the full depth that your body allows. And there is no reason not to try and continue developing that mobility and those ranges of motion to squat deeper and move more freely…while maintaining the proper form and posture.