The usual recommendation is to check your saddle twice a year. This is because your horse's shape is likely to vary throughout the year, either due to changes in their weight or fitness levels. The age of your horse is also a factor.
Reflocking is usually recommended every 2-4 years. Additionally, the new saddles should also be checked after 20-30 riding hours, as after a few rides, the wool settles and a reflocking might be needed.