I've never tried to care for a genuinely abused horse. However, both of the horses I have now had some rough treatment prior to arriving, and I didn't understand at the time how much TIME it would take for them to recover mentally.
Mia had been used for some endurance racing, then not ridden for a year, then given to a charity, and sold as a horse for a 12 year old girl. She is a very dominant mare, and she was placed in a pasture with a bunch of other horses, including a few that weighed 1500+ lbs to her 900 lbs. She wouldn't give in, so she was beat up regularly by the herd. She also had no desire to be ridden by a 12 year old, and a few months later she was returned to the charity with bite marks and 150 lbs lighter. And she was sold to me, a newbie, as a good horse for a beginner rider!
We already had a horse, and she had been Mia's corral mate for 2 years prior to being donated - odd chance, but we figured it would all be hugs and kisses right off. Lilly thought so too. She was dancing around before we got Mia out of the trailer. So we turned Mia loose...and she darn near tried to kill Lilly!
We hastily build a new pen for Mia, and let them get reacquainted - first with a 5 foot gap between corrals, and then over a common fence. A month later, they were pals again. And Mia gained her weight back quickly. But for 4 months, she would break into a heavy sweat just standing still. She had multiple bouts of colic eating the same feed regime her original owners used (she had papers, and we had the name of the original owners from that. They were shocked when we emailed them pictures.)
Meanwhile, in my ignorance, I started riding - a brand new rider with very limited experience followed by a 30 year gap without touching a horse. And I rode English. Until she bolted during a dismount, and I landed back first on some rocks. 3.5 years later, my back still hurts 4-5 times a week.
It took at least 6 months for her to get over most of her anxiety. Maybe a year to get back to feeling fairly confident. The problems I have with her now are caused by my lack of riding skill, but I've been working that issue hard for close to a year. In Nov she will get a month of training by a professional who has helped my family enormously, and by Dec I hope my riding skill and her mental worry level will match well enough to resume riding after a 9 month break. If so, then only 4 years of work and riding will have gone into it...
Trooper was bought from a ranch in Utah owned by an old friend. However, it took 6 months for us to get a place ready & arrange transport for Trooper, and he was loaned to a ranch in Colorado that promised my friend to feed him and ride him regularly.
Well, they did. They also spurred holes about 2" diameter in both sides, wore a hole in his withers with a bad fitting saddle, used him for cutting although that was the only restriction my friend had made on his use (at 800 lbs, he's a bit small for it), and our farrier is certain he was roped and thrown for shoeing - for no reason. My friend offered to cancel the sale, but we went ahead.
And yes, when his sides healed, we rode him. Which was OK as long as he didn't think he had done something wrong. If he thought he did something wrong, he would panic and try his best to dump his rider.
Eventually, I gave up and stopped riding him. I started looking to see if anyone would be interested in taking him on. Both our trainer and farrier thought he was potentially the best horse we owned, and we would have given him away to a good home, but no one wanted him. He went 8 months without being ridden - just lots of handling, or sitting on the fence and talking to him.
Since no one wanted him, we sent him to 5 weeks at the trainer's place. She started him over at the beginning, treating him like an unbroke horse. It was 4 weeks before she mounted him. One week later, he was ready to come back.
He was still nervous, so he went on the "You can do no wrong" program. Lots of short rides. He could trot as much as he wanted - that being his way of being nervous. At first, most rides started with 15 minutes of trotting. Then 10. Then 5. Eventually we did walks followed by trots. My youngest daughter started taking lessons on him.
A year later, he is an outstanding horse. He's in the process of teaching my daughter-in-law and I how to canter without bouncing. His goal in life is to understand his rider, and do what his rider wants. My youngest daughter rode him last week, supposedly because she wanted to try cantering. Trooper refused to canter, although he loves it. When I asked my daughter later, she confessed she got nervous and didn't REALLY want to try cantering. So Trooper cantered with me, refused to for her, then cantered with my daughter-in-law during 3 sequential, 20 minutes each rides. He picked up on what my daughter REALLY felt, and behaved accordingly.
As I look back on it all, I've decided that if I ever buy a horse again, I'll start by giving the horse a vacation to get used to his surroundings and the other horses. Maybe a month, maybe much longer.
If he had been genuinely abused, maybe 6-12 months with nothing more from me than handling, and maybe some easy round penning near the end. A horse that has been treated badly takes FAR longer to get over it than I had imagined. But they CAN recover. With time. LOTS of time.
BTW - the previous owner of Mia & Lilly had taken Lilly in from abuse as a one year old. She said Lilly was afraid of humans, so she just spent time talking to her and handling her. For 5 years. They had other horses for riding. When we got Lilly, she was a super sweet, eager to please Arabian mare - ready for training. In the time I spent riding her, she never tried to buck, didn't bolt, and gave every sign of enjoying the time with a human.