Stupid question, but what's the average barrel life of a chrome lined 5.56 barrel? Not to the point where accuracy starts to drop, but to the point where it's unsafe to shoot.

Ummm.  
A lot.  IIRC the military says a minimum of 40,000.  
You will probably never wear a CL out unless you have a FA tho.

Wear will not usually make a barrel unsafe.  The throat of the rifling will wear away until the bullet has to "jump" to the point where it will engage the rifling, and accuracy will go away as this progresses.  In my military unit we had some vietnam- era captured AK's for training rifles that had virtually no rifling left, and they still functioned. Just woulden't hit the same place twice.  
actually, AR barrels are pretty tough, as even extensive full auto shooting didn't hurt them too bad.  High heat accelerates wear considerably.  Usually, what would happen was wear would eventually expose bare steel in the throat of the barrel, and then corrosion would trash the barrel.  We worked in salt water, so this didn't take long.