The most important thing to consider is their execution. Squats should always appear fast and explosive. If you are training at such an intensity that they appear slow and laboured then you had doing them too heavy.
The right stimulus for squats can be found at as low as 80% of your 1RM. So if your best backsquat is 150kg, then 3s on 120 is a good strength training minimum. Squats are used at the end of a training session to finish off your legs if you have some steam in the tank left over. If leg strength is not your weakness a simple 4 set of 3 reps at ~80% is a good level to aim for. Try to squat at least 3x per week.