The severity and density of residue in a chimney will vary depending on many factors most obviously the choice of fuel, frequency of use, location, installation etc.
NACS recommends sweeping as often as every 3 months, depending on the type of fire/fuel. This will help reduce the risk of fire and carbon monoxide poisoning, help maintain the efficiency of your appliance and probably the life of stainless steel liners.
NACS recommends the following frequency of sweeping: