There are various things which affect how often sheep need their hooves trimmed. Certain breeds do tend to have 'good' feet - black hoof is a bit harder than white. If the ground is rocky or there is some hard standing then the hooves are worn down whereas on grass alone (which we have) they can need to be trimmed more frequently. Persistant wet weather is bad for feet as it keeps them soft and allows for overgrowth with inclusions of mud. Long wet grass tends to cut between the toes and can lead to the development of sores. If some stock have foot rot already then it will spread to the rest of the flock, so the feet will need to be treated more frequently, and any with rot isolated. Some individuals inherit bad feet, or a tendency to bad feet.
So there is no set frequency for trimming feet as it depends on your individual circumstances. We check all our sheeps feet whenever we have a big gather or at shearing, but we also keep a close eye on them all and catch and treat any with a limp as soon as it appears.