I ran my fireplace with 1.5-2 cords per year of dried birch and had the cleaned after about 5 years when we first moved in. I used this Kathite stuff that used to be used in industrial coal fired plants from Canadian Tire where you throw 2 scoops into the fire. The guys came out to clean the chimney and said it had next to no buildup. I do the cleaning every year late summer just to get the receipt for insurance purposes and piece of mind.
http://www.kathite.ca/