How long can a hoof go poorly trimmed?

My friend's horse was recently diagnosed with an atypical case of ringbone. The horse is 16 years old, so not really old in the whole grand scheme of things. The vet thinks the main cause is this horse's hooves are very low in the heels and the broken back pastern angle contributed to the problem. Poor guy now has arthritis and a limited riding career left.

It got me to thinking about how long any horse can go with poor trimming before it causes permanent damage. My own horse had 7 farriers over the past 5 five years before we finally found one who could properly trim a foot. He spent 8 months on average with long toes, then short toes, then underun heels, overly high heel club foot, etc. Are there any studies on how long a healthy horse can have poorly trimmed feet before it starts to cause lasting damage?
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Good question.

I once worked on a foundered horse that had extremely high heels and pretty serious rotation. I don't know (nor did the owner) how long he'd been neglected, but he was in real bad condition, hoof-wise.

I was about 22 and thought I was smarter than the vets and everybody else, so I wanted to fix this fella by myself with no x-rays or help. So I cut his heels down and tried to fix it all in one trimming. Well when I got done, this poor horse stood on his tippy-toes. His heels wouldn't touch the ground at all.

So I put some wedge pads and shoes on him so he could walk and told the owner let's go to the vet and git x-rays. We did and we found P1 and P2 were fused together on both front feet. He'd been foundered for a long time, not treated, and I didn't know but always wondered how long it takes for calcium deposits and such to set in

If the horse has plenty of gravelly footing then his hooves will self trim to best support his needs.