I use a chiro for my lads. Bandit isn't in heavy work, he's more a pasture ornament than anything else at the moment due to the fields being in muck and me having nowhere to ride
I'd only have him out every 6 months, if he is in the area he calls into me to have a quick look and see if anything needs adjusting but rarely does he need extra work. The first time he did Bandit, he had to give a thorough going over as Bandit's body was out form the atlas to the pelvis - years of wear were very evident.
At work the chiro is out every second week, but the horses are in full training (racing). While speaking with him the last day he mentioned that this year he is finding significantly more horses are "out" due to the heavy ground.
I think the trick is to find a GOOD chiro... and I don't just mean somebody who is good at the job... I mean somebody who isn't going to play you for a fool and take money every 10 ten days because they can. I have a good relationship with our chiro, both through work and outside work. There is about a 6 week waiting list for him, but if I called him tomorrow and said I was unhappy with Bandit, he'd be out fairly sharpish. It's the same with blacksmiths, chiros, dentists, ect - a good relationship is key. SOmebody I know got my chiro out and was amazed when he charged half the price as others, and told her not to get him back for 3 months. She was shocked as her last chiro had done numerous treatments in a short space of time and charged a small fortune.