There is a way to quantify it, by measuring brain farts per 1000 miles. I'm not sure what the threshold should be, or if some brain farts should be counted as double.
While that might seem like I'm just joking, the reality is one needs to ask how many times they have just gotten lucky not to hit anything, or get into trouble. Those near misses should start to tell you something. If you are asking the question, maybe that is the time to start keeping a driving log. When you see the rate start to rise, or you can't remember where you put the log book, then maybe its time to retire from driving the big rig.