How often your cat needs to be groomed needs to take into account a few different factors. The breed, age, weight and health. Whether your cat is indoor or outdoor and probably the most important factor, what grooming are you doing at home to assist them with their coat. If mats are forming before your next appointment you need to think about coming sooner between appointments.
Persian/British Long Hair/Scottish Fold - Every 6 to 8 weeks (Dilute coats blue/cream/strawberry blonde/sand are more mat prone to their counterparts black, white and ginger)
Ragdolls/Maine Coon/Norwegian Forest Cats/Siberian - Every 8 to 12 weeks
British Short Hair - Every 12 to 16 weeks
Domestic Long Hair - 8 to 12 weeks (If they are part mix of any of the above breeds they will need to come sooner)
Domestic Short Hair - Twice a year (Spring shedding season March/April/May - Autumn when they shed again going into their winter coats October/November)