Every rep must be to parallel no cheating. The proviso here is that your lower back must not round. If your lower back rounds at the parallel depth, try a wider stance with toes turned out more. That stance change may enable you to go a bit deeper without your lower back rounding. With those rules established you need some mental techniques to get you through each set.
Squats are largely a head game.
In watching many people on a 20-rep squat routine I've seen how different characters cope with the intense discomfort, but there are some things that can help all types. When the temptation to put the bar back is becoming almost unbearable here's a few things you can try. These principles can be adapted for any exercise. Retreat This is where experience of meditation can be helpful. There is a place inside us where we are the witnesses of what is happening to our bodies, and retreating there can lessen the discomfort of squatting.