This depends a lot on how much you use your fireplace or stove. The National Fire Protection Association says, “Chimneys, fireplaces, and vents shall be inspected at least once a year for soundness, freedom from deposits, and correct clearances. Cleaning, maintenance, and repairs shall be done if necessary.” So, even if you don’t use your chimney a whole lot – birds, squirrels, raccoons and other critters may have been using your chimney making it unsafe to use without clearing out the accumulated debris from nesting activity.

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The Chimney Safety Institute of America recommends that open masonry fireplaces should be swept at 1/8" of sooty buildup, and sooner if there is any glaze present in the system.  This is considered to be enough fuel buildup to cause a chimney fire capable of damaging the chimney or spreading to the home. Factory-built fireplaces should be swept when any appreciable buildup occurs. The logic is that the deposit is quite acidic and can shorten the life of the fireplace.

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Q. I heat with gas. Should this chimney be checked too?

Without a doubt! Although gas is generally a clean burning fuel, the chimney can become non-functional from bird nests or other debris blocking the flue. Modern furnaces can also cause many problems with the average flues intended to vent the older generation of furnaces. We suggest you check the areas on gas and carbon monoxide for more information.

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Sources: http://www.csia.org/about-csia/faq.aspx

https://www.mychimney.com/blog/about/how-often-do-you-need-to-clean-your-chimney/