As the tyre tread depth is directly related to the driving safety of the tyre, it is very important to check and monitor this. You need to know that the wear is higher if you are an aggressive driver and it is higher on the driving axis of the car. So this requires that you need to rotate the tyre between front position and rear position. It is recommended to do so every 8000 km or if you change between summer and winter tyres, this would be the time to mark the tyres so that they can be changed for the next season.
In addition to just measuring the tread depth, it is important to visually inspect your tyres to make sure that they are free from any visual damages. The sides of the tyres can be damaged and can cause a blow out. If the tyres look ok, they probably are in good condition and you can wait until the wear on the treads has reached its critical level before changing.