Fly Ash ,Flyash, Dust

I asume that flyash is what is created when the stove is shookdown. Happened to have door open when I did it this this am and noticed an incredible amount of dust heading out the flue.
I assume this is going to deposit itself at the point where my stove pipe makes its transition from horizontal to vertical where two 90' elbows are joined at 90'.(form offset)
I think most of the dust that my woodburning made ended up here to the point to which it almost closed off the chimney last spring.
How often should expect to have clean the pipe out? With a hot fire dailey to "cleanout burn" the chimney with wood it wasnt much of a problem maybe twice a heating season. How about with ash from the coal?

I have to clean mine out about half way through the season. Usually we get a warm spell for a day or two when I don't burn the stove and thats when I do it. I have a 90 coming out the back of the stove and a 90 going into the thimble. The horizontal run through the thimble is where I get the build up of ash. I can usually tell by a slightly diminished draft that it needs to be cleaned out. From reading the ideas on this forum, I will be installing a T where the thimble 90 is and capping off the one end. This will allow much easier inspection and cleaning than dis-assembling the 90.

I once thought about a "tee" (would solve a lot of cleaning probs.)but was afraid to try it burning wood because it used to get so hot it would sometimes turn red hot. It was also gathering spot for the burned up creosote dust and ashes from the furnace.
So far am very impressed with how much cleaner the whole thing is with coal over the firewood.
Skeptical friend said I would switch back to wood within a week....He was wrong...lol

I also find burning coal to be cleaner than wood. Not only in the chimney cleaning, but mostly the debris left behind from bringing in the wood that gets stacked in the basement. Two other advantages from my point of view....Its easier for me to bring in the 5 gallon buckets than to carry in armloads of firewood. Its a lot easier on my back to have two ton of coal delivered and dumped rather than cut up, split and stack the firewood. I still burn wood just because we have blow downs to use up, but each year I am burning more coal and giving away more firewood.