Being basically lazy and not wanting to hold a wet blanket up against a red hot fireplace to extinguish a chimney fire, I choose to keep the chimney clean. Here is a good rule of thumb. Have the chimney inspected and cleaned every two cords of wood. If you use the fireplace only occasionally, still have the chimney inspected yearly.

It is difficult for a homeowner to thoroughly inspect a chimney. You might want to look up in there to see how the creosote is doing. With a good bright flashlight and a mirror to look around the corner created by the smoke ledge, try to look up the chimney. If the metal is shiny silver or the ceramic flue liner is bright pink, it's clean. If it is black and furry-looking up there, time to have it cleaned.

Check out the firebox as well. Poke around in there looking for loose bricks or metal that is thin or warped. If you find either one, have the problem repaired before you build your next fire.