How long before rebuild on a 3406c

For most local construction type trucking, most go by engine hours. Depending on the engine, the capacity of the crankcase, and how much off road work is done would be the main factors determining engine maintenance.
If you are 12k hours now, if oil consumption is minimal, power is good, etc., I imagine you are 1/2 way to need of an overhaul, maybe longer. Be meticulous on your preventative maintenance, use the highest quality oils and lubricants you can afford.

 

@wore out

 

If the maintenance was decent id guess you have quite a bit more time left in it…. A bearing roll probably wouldn’t be a bad idea but depending on who does it and how clean they keep things it has its risks
I have a 3zj that I pulled out of a truck with 800k and it had never been opened (everything had matching date codes) it was originally an over the road truck and later a local truck so probably had fairly high hours. Must have been maintained pretty well because everything inside looked great (bearings were showing wear but looked normal) I put bearings and injectors and resealed some stuff and put it in another truck.