I think it depends on the horse. Some horses can go longer than others and I would say between 6 months and 1 year. I'd usually have them done six months later then ask the dentist if they could have gone longer. If so, try 9 months next time and see how you get on.
I do six months as Cairo is an OAP and Chancer a youngster. My dentist is excellent if he comes out and does nothing, no charge. He does come out and do all the yard at once for this reason.
My dentist asked to see Morris every 6 - 9 months as she reckoned he'd had a bit of work done on them when he was younger and he'd need frequent visits to keep them nice
Totally depends on the horse, but its usually about a year.
Depends entirely on the individual horse; Usually horses aged between 2-8 every six months and then 8-15ish every 9-12 months and then 15 onwards every 6-9 months