I repot all of my slippers at least once a year into a bark mix (supplements depend on root characteristics and my history with the plant). I find that I often need to repot the plants in larger pots sooner that the smaller ones. I think that despite my best efforts to water in accordance with pot size, the larger pots stay just a bit wetter and so break down more quickly (~8-9 months). I have not had any rot problems though, so I think this is OK as long as I keep close tabs on it.
Other dryer growing genera can usually last about two years for me, provided I start with a large enough particle size to keep the air flow high.
I repot my paphs after 1-1.5 years in my CHC mix, multiflorals 2-3 years. Phrags are in bark, and I repot them every 2-3 years, unless they outgrow the pot. Phrags are frequently forgotten and left in the mix for 4 years, but they seem to love wet rotten bark....I water directly into the pot....every other day for the underlights growers, once a week in winter, maybe twice a week other times, a little more in summer if its hot and windy...for paphs. Phrags are always wet, at least 5 times a week. Take care, Eric