I have been around horses on my own family's property for 27 years and I don't call out the vet for anything less than catastrophic. Most your typical cuts, bumps, lameness, etc, etc, I can handle myself. Even some of the nasty ones. I know how to do stitches in an emergency, even though I prefer not to.
I pretty much follow the same plan as Apachie. Minor colic, the vet gets a phone call to let them know I have an issue, but I'm treating it and they will only hear from me again if something goes wrong. That gives them a heads up that they may have to make an emergency farm-call. Though it has been so many years since we had a bout of colic, I don't really worry about that one too much. Eyes, yep, don't screw around with that. Choke is another thing I don't wait for, that's also an immediate vet call.
If it's something that I am not entirely sure about and am not sure whether to call the vet or not, my first call is always to my Dad. He's been around very large numbers of horses for about 60 years so, if something does happen, odds are he's dealt with it or seen it at least once before.
We keep a bottle of Banamine and Penicillin in the fridge in the house, plus plenty of needles/syringes, I know how to give shots when the horse needs it (works much quicker than the paste or pills).
Pretty much the only thing our horses have seen a vet for in years is your basic care that a person cannot do for themselves; teeth floating, castrating, coggins, and Flash got to go to the vet this winter when he developed sudden diarrhea and began to drop weight quickly. Dobe got to go a few years ago to have some melanoma's removed, but that is pretty much the only vet calls we have.