During the winter, if the horse is a trim, usually it's scheduled for every 8 weeks which then will shift to 6 weeks once it's shod. Some horses need a tighter shoeing schedule of 4 to 6 weeks - that gets based on the horse's growth and its use as well as the wear on the shoes. Unshod horses get a short interval as well during warmer months when there's much more growth; usually a 5 to 6 week schedule works for the horse getting a trim.
When a horse is scheduled largely depends on the individual growth rate and use of the horse. Horses that are outside in the winter tend to grow more slowly and are on an 8-10 week schedule while inside horses continue to grow faster and remain on a 5-6 week schedule. During the summer months, shod horses tend to be on a 4-6 week schedule and barefoot horses are on a 6-8 week schedule. Horses suffering from lameness issues are put on a 4-5 week schedule.
I like to see my horses ever 6-8 wks based on anticipated growth rate and wear and tear. I will fire a customer that does not stay on schedule. I will charge extra if they get off schedule so much it causes me more work. I know there has to be some flexiblity but I do not like being the member of the horse maintenance team that worries the most about a customers horse.