Just bought an M&P 15OR...I know its new, but I'm just wondering what's the typical barrel life on this gun using brass ammo...NO STEEL.   And also how do you tell when your barrel is getting to its end life??  Thanks

unless you are competing at a lot of competitions, you will die before your barrel gives up if you maintain it properly.

15 thousand to 45 thousand rounds as a rough guess.

Unless you damage it, it's dead when you decide it's not shooting tight enough groups for you.
A benchrest shooter will replace barrels much more often than most because they are worried about ultimate accuracy.  A person like a friend of mine with is close range training rifle didn't even care until it started keyholing. And the baffle strike.

Most people, shooting at typical ranges, up to say 400 meters maybe, will be fine for quite some time. Is 3 MOA good enough for you? 2? 6?  It's a call YOU make.

Are they chromed lined?  I have never shot out a barrel but I have read 15k to 20k rds.

Steel is GTG.

I wore out a chrome lined 20" govt profile Bushmaster barrel, that I bought new in 2004.... in 10,000 rounds.

I noticed I was having trouble hitting clays with it... and put it to paper.  The group size more than doubled.... and you could not see any rifling just past the throat.