I would say, there isn’t a norm, and what you should do is really going to be a function of who you have available to you in your area.
The most important fitting is the first one. Get a saddle that basically works for your horse, and for that, especially if it’s a hard to fit horse, it may even be worth a fair amount of travel to get it in front of a fitter and a sampling of saddles, or to pay someone to bring inventory and expertise to you.
If you find someone good, follow their advice in a way that makes sense to you and works for your particular logistics. I am really skeptical that anyone can do adjustments remotely without seeing the horse. Sometimes we have to rely on these long distance possibilities and there I think you just do the best you can and listen to the horse. Learn all you can about how to assess the fit so you can be smart on your own, once you have a saddle that works.
It’s lovely if you’re near someone with expertise who can come and look once or twice a year. That’s really not typical.
I’d also say that I know a few people who have tried the custom route and those seem if anything more finicky and troublesome than an off the rack brand that has been adjusted, unfortunately.
If you live in a REALLY remote place, there are some entities that I have heard can work with elaborate tracings and measurements to build custom saddles.