Welcome cjk.  There is little to no ash discharging from the flue.  You will get some embers and small bits of incandescent material exiting the flue occasionally during a burn, and that is why the 55 gal drum is there to catch them.  Ash does build up in the primary fire box, but amazingly little.  I shovel out less than a 5 gallon pail's worth every other week.  It takes about 2 minutes, and with the draft fan on you get no dust/ash in your face while doing it.  It is fantastic.

 Have you ordered your GARN already?

 I heated with a soapstone stove for about 20 years and have been using the Garn now for almost 5 years.  The difference in the amount of ash is large. How often you need to remove it remains a function of how much wood you are burning but my experience is about 10 days in cold weather heating 4000 sq ft all radiant.

 The barrel is there not so much to catch ashes but more to serve as a catch "basin" for sparks and the occasional hot ember that gets pulled all the way through the Garn. I advise the customers who use that type of venting to put about 12" of water in the bottom of the barrel to extinguish hot pieces that are discharged. 

As a general rule wood that is completely combusted will leave an ash residue of .4% of the weight of the wood being burned. In other words, 100 pounds of wood when burned completely will leave a residue of about 5 ounces of ash. The Garn does a pretty good job of complete combustion so ash cleanout is not much of a factor. One of my customers told me recently that he doubts the total amount of ash he has removed so far this year from close to 13 ricks (face cords) of wood would fill a 20 gallon wastebasket such as found in a typical kitchen.