How often should I clean my fireplace when I burn pine for firewood?

I live in a rural area and I have a ton of pine trees on my acreage. I have been told that pine is a very "sappy" wood and that it should be fine if I "burn seasoned wood that is cut properly" (asked my chimney sweep) and if I use caution. I know that you are suppose to clean your fireplace once a year, but do you need it more often when you burn pine? How often?