The answer is 'it depends' don't Ya hate that??
It depends on the amount of coal consumed.. the amount of fly ash will vary with the quantity of coal burnt.. I'd say at least once a season, or it culd be as often as every month or so..
From the standpoint of safety I'd just install the baro damper at the top 'ELL' or elbow location, then you can very easily look in through the door of the damper [held open with a finger] using a flashlight and see how much ash has accumulated..
I didn't mention, I'd also install a '
tee' at the exit from the stove, if this is a back vent or bottom vent stove,, if top vent, there will not be an exit elbow.. Then again, cap the extra leg of the TEE' and use this for a conveinent cleanout.
If you do end up cleaniing every month or so,, it will take only about 10-15 minutes to perform the cleaning,, with the capped tees it is really fast and easy..
Go through the 'Baro damper thread' and look for the photos that member 'wood 'n Coal' has there,, you will see that it makes access really simple.
Hope this helps,, Greg L