How often you need to clean your chimney depends on so many different factors: the type of wood you burn, the way the chimney is built, how often you use the fireplace, and how well your chimney really vents. It's impossible to make an accurate, specific recommendation on how often you should clean the chimney. A far better approach is to be aware of indications that your chimney is in need of cleaning.
Situation 1: You're in a new home.
If you just moved into a home with a chimney and do not know for a fact that the chimney was cleaned within the last few months, you should have it cleaned before you use it. When a chimney sits unused for a while, birds can build nests in it or larger debris like leaves can end up inside, increasing your chances of a fire. Plus, since you don't really know the history of the chimney's use and maintenance, you don't know if there is any creosote buildup further up the chimney. Having the chimney professionally cleaned will also give you a chance to verify that it is properly ventilated before you light a fire and put yourself at risk of carbon monoxide poisoning. (You may have to pay the chimney cleaner an extra fee to inspect your chimney for ventilation issues, but this is worth your safety.)
Situation 2: The chimney has sat unused for a year or more.
If it has been longer than a year since you've used your chimney, you should also have it cleaned in case there are birds' nests or other debris inside.
Situation 3: You note creosote buildup when inspecting the chimney.
Every couple of months, take a few minutes to check your chimney for creosote buildup. This flammable substance accumulates on the inside or your chimney when wood does not burn completely, and it greatly increases your risk of a chimney fire.
To check for creosote, wait until the fireplace and chimney are completely cool. With goggles on for safety, reach up into the chimney, above the damper, with a poker. At the same time, shine a flashlight up into the chimney so you can see the sides clearly. Scrape the side of the chimney with the poker. If what you scrape through is any thicker than a piece of paper, you have creosote buildup. Don't use the chimney again until you have it cleaned.
Your professional chimney cleaner may be able to make a more specific recommendation as to how often you need your chimney cleaned based on your individual chimney and how your family uses it. For more information, contact local professionals like Country Corner Chimney Sweeps.