Unfortunately, there is no rule of thumb to say how often you should clean your chimney. Creosote forms whenever wood is burned incompletely. If there is not enough oxygen in the smokey fire then lots of tar vapors are emitted. These vapors can condense inside the flue and settle there. This may lead to chimney fire.
You need to check you chimney to find out whether it needs cleaning or not. First make sure there’s no downdraft from the chimney. If there is downdraft then open a door on the same floor as the fireplace until the flow is reversed and the air flows up. Now gear yourself up. Take goggles, dust mask, flashlight, and fireplace poker. Scratch the black surface on top of the smoke chamber. If the groove that you scratch in the creosote is paper thin then you know that no cleaning is required. If it’s 1/8 inch thick then you need to make appointment for cleaning soon. If it’s is ¼ inch thick then you should stop using the fireplace until your chimney is cleaned, else chimney fire may occur.
There are four places to check for creosote: the top of the firebox, around the damper, the flue and in the smoke chamber. Four types of creosote can accumulate. The first type is a light dull gray, black or brown soot. Then there is a black granular form that can be removed with a chimney brush. The third type is a road-like tar coating and then there is a shiny, glaze-like coating. You can reduce the amount of creosote buildup in your fireplace by making sure that there is enough combustion air that produces clean burning fire.
When you find out that your chimney requires cleaning, you should call your local Chimney sweep service.