It will take three fires a day, starting very small, for a total of 7 days, and ending with the firebox full of firewood, to "cure" all the mortar. The place I bought the doors and refractory slabs from, Solid Rock Masonry in Duluth, MN, gave me a helpful users guide that I'm following.
It's looking real good. There's a strong draft from the chimney; the fires just jump. The design of the firebox door has a lot to do with that. In the video I put my finger into an air intake slot, located below the glass door panel on the door frame. The door frame is hollow steel tube so the intake/combustion air goes up the sides of the frame and then across the top of the frame where it enters the firebox. Once lit, the flame goes "looking" for air and is drawn, by design, upward, giving the fire a strong reason to head up. This design also, according to particulate/emissions tests done by Solid Rock Masonry, gives a much "cleaner" (and hotter) burn.
There's quite a distance traveled in internal channels before the smoke actually makes it to the chimney. From the firebox its five feet straight up, then seven down the sides of the firebox. Then around two corners under an eight foot bench, and finally up the chimney through the second floor and attic.
It's been a long time coming. I first heard about masonry heaters almost 30 years ago and the idea really stuck with me. I never lived in a place where I could build one. Here in NW Wisconsin is the perfect spot.