I've had to back off a bit lately due to crippling cramps in my upper back
Up until recently, I had always thought that I had been training hard and at my full capacity.
Everything is relative.
I was doing between 10-14 sessions a week for a good month and maxing out squats, press and deadlifts pretty much every session.
At this point, I'm finding ways to get myself to be able to train harder.
Pulling from the floor every day makes my upper back cramp up, low bar squats inflame my SI joint - but I can kill it every day with front squats, shrugs and rows... so I have found the exercises that I can work to death without dying myself
I'm pretty much of the opinion now that most people who are training for strength "should" be gradually increasing their volume, average intensity and frequency until they physically can't increase it any more due to cramping, injuries or time constraints and then backing off.
To put it bluntly, if your body isn't screaming at you to stop then you've probably not been anywhere close to your limit.