One obvious answer is when the groups really open up, but as far as “Preventative Maintenance”, does anyone have a comment on when to change your barrel?
I shoot mostly commercial (non-wolf) 55Gn. FMJ if that matters.
Assuming its a good quality, chrome-lined barrel and you take care of it, there’s no reason it shouldn’t last 20k+ rounds before you should become concerned about throat erosion. Even then, I’ve seen barrels shoot quite a bit more than that before they needed to be replaced. At the current cost of ammo, you’re talking about several thousand dollars worth of 5.56x45 before you have to spend $250 for a new tube.
Match/competition barrels are a completely different story, of course.
That pretty much sums it up. Change it when you your groups start to consistently start to open up. Clean it properly & it should last you a long time.