The answer is at least once a year.
More frequent cleanings may be needed based upon:
1. How often you use the fireplace or wood stove.
2. How your fireplace and chimney are constructed.
3. How you manage your fire.
4. What type of wood do you burn.
5. How well seasoned the wood is.
6. How often you let the fire smolder itself out
7. What the weather is like.