Hejsa igen
For range use, a properly maintained AR15 will practically last forever. I think the life of a barrel is probably about 10000 rounds, and obviously common parts like springs, gas rings, bolt cams, extractores, etc. might need to be replaced over time as well, I can't really think of anything that would just "wear out" - I guess, if you are unlucky, an AR (or rather M16) that is used a lot in full auto MIGHT start to deform the pin holes in the lower, but my impression is that is very unusual and very rare. Even with "hard" use - shooting thousands of rounds and treating the rifle hard, with regular maintenance, a AR will easily last decades.
Hell - the Danish army was using M1 Garands from World War 2 up into the 1990's (called gevær m/50) - and some of those had been used very hard over the years. THEN, those same garands were returned to the U.S. and sold to civilians, and they
still work perfectly. That's over 60 years of military use, and the rifles function perfectly.
Here are some of my garands - all of these are original WW2 Garands, and at least three of them were issued to the Danish army from 1950 to the mid 1990s.
If they can last 60 years in military service and still function perfectly and shoot accurately, I think you can EASILY assume decades of proper function in an AR if properly maintained, even if used hard.