The simple answer is: The National Fire Protection Association Standard 211 says,

This is the national safety standard and is the correct way to approach the problem. It takes into account the fact that even if you don’t use your chimney much, animals may build nests in the flue or there may be other types of deterioration that could make the chimney unsafe to use.

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

Chimney Cleaning… Here’s what we do

• Thorough brushing of the entire flue, top to bottom
• Inspect for internal chimney fire damage, cracks and/or degradation
• Clean chimney base, fireplace smoke shelf, damper and grates
• Rotary clean smoke pipe and fireplace smoke chamber
• Clean stove/furnace interior, baffles, catalytic combustors or secondary burner chamber, gaskets, firebricks and fans
• Inspect chimney exterior, flashing and crown for weather tightness
• Perform minor repairs when possible during scheduled cleaning service
• Major repairs identified with cost estimates provided