There is no firm rule on how often you should clean a chimney. You cannot base a schedule by the number of years of service, nor can you break it down to the number of fires you have had in the fireplace. An inspection of the chimney is required to properly determine if a cleaning is required.
Use your fireplace poker or other metal device to scrape away some of the black substance (creosote) from the fireplace lining. If the creosote buildup is paper-thin, no cleaning is required. If the buildup is 1/8 of an inch, you should schedule a cleaning. A buildup of 1/8 to 1/4 inch requires an immediate cleaning. If there is a buildup of more than 1/4 of an inch, you should not use the fireplace until it has been properly cleaned. (Make certain to wear eye protection and a dust mask when making your inspection.)
Creosote is the residue of the wood you burn in your fireplace. Different types of wood will leave a variety of creosote buildup. For instance, pine will cause a rapid creosote buildup and should not be used in your fireplace.
A buildup of creosote in the chimney will reduce the draw of the fireplace and lessen its efficiency. If the creosote buildup is significant, it can catch on fire resulting in a flue fire. These can be very dangerous and are easily avoided by keeping your chimney clean.
There are a variety of do-it-yourself cleaning instruments and kits a homeowner can use. You should follow the directions in the kit and make certain the kit is for your chimney type (metal, brick, rock). Always make certain the fireplace and chimney are completely cool prior to cleaning.