Most road tires will have 5 to 6 millimeters of tread on the front and similar tread on the rear tire when they’re new (depth varies slightly for wider/narrower tires). Motorcycle tire manufacturer’s tests shown tire wear is quite linear, but as the tire gets towards the last millimeter, it actually slows very slightly. Also with the latest compound technology, performance reduction as a tire wears is negligible unlike old tires which could get quite nasty when they got low on tread.
The legal minimum is one millimeter of tread (well, never seen a police officer actually checking the tires). Don’t assume a deeper tread means longer tire life. This is not the case; it is down to the mix the compound.
If anyone suggest fitting a non matching softer front and harder rear tire, don’t do it. Tire manufacturers have thought of this and there is almost no front tire available that has the same compound as the rear due to the different roles of the two tires. The stiffness and friction coefficient of the front compound is calculated to react and absorb the push of the rear tire reducing over-steer and allowing cornering precision.