Putting an averages on a set of tires is not easy to determine, the depth is one of a few indicates, the other is the general condition of the tires themselves, are they crowned or cupped, are the missing pieces in the tread or scraped up on the side of the tire. Were they rotated at all during the life on the truck?
I had no choice but to replace my steers, they were cracked and a mess. I put the best I could get, Goodyear G395 and happy with them. It will get another alignment towards the end of the year.
The life of the tires depend on a few things that I mentioned but as Rich asked, what type of truck are you talking about matters somewhat as the most important thing.
The factors that I am told make the difference is the design of the truck in relation to the axle alignment, the PM that the truck receives and the weight dispersed on the chassis in an unloaded state. This also includes properly inflated tires – depending on the manufacture’s recommendation on the load of the tire.
Next to this is the tires and material in the tires. The material used today is different from 10 or even 5 years ago, things are improving all the time. Also don’t be cheap with your tires, get the best ones you can afford.