You can probably get an idea about the DL depth in my coop by looking at that vid but I've never had it any deeper than maybe 10-12, if that. Usually hangs around 6-8 in. on average, with dips even lower in the summer months when the composting really kicks in high gear.
I can understand the questions as I am constantly learning about this DL too. I would consider last winter my most successful winter yet with the DL and it was after I had changed many things in my coop. I have a hoop coop, so ventilation is always pretty good but I provided even more with this last coop tweak and it was also the very first winter I didn't use a single bale of wood chips. Eureka! The increased ventilation at certain points in the coop and the very minimal existence of the wood chips were important. I thought I had made a mistake by putting down sawdust at the lowest levels under the roosts but that turned out to be a happy accident...they served to hold moisture in the bottom levels there, while the leaves and such seemed to ride on top. In the vid you can see that I had to really dig to get that level up to the surface, so I know the chickens can't disturb it.
Before last winter I thought I had great ventilation....but after doing a winter with the whole top of the coop door being open to air and various large spaces at all levels open to air, even in -17* weather, I was simply amazed at how much more warm and dry the whole coop and, especially the DL, could be. So much so that I took my old reliable Coke thermometer and hung it next to the roost so I could see what the temps were at roost level at night. I was amazed! I think that additional ventilation took the warm, moist air from the DL and just constantly moved it up past the birds and out the top of the coop. I could feel an immediate difference when stepping from outside into the coop and I never had that experience before.
I think it all comes down to opening up various large areas of ventilation at all levels of the coop and then just adjusting them to get the right airflow going at any given time. Then choosing materials that are more likely to keep composting even when temps are extremely cold. I never even saw frozen poop this past winter, but in previous winters I would come to the coop in the morning to find the poop had frozen on the DL or even chunks of frozen DL(mostly the pine shavings) and even frost on top of the leaves in the bedding. Last winter was our coldest yet and none of that was in evidence. Nice and moist underneath, dry and light on top, 10 degrees warmer at roost level all night long. That DL was even warm enough to keep my fermented feed from freezing...when we got down to -17* I was having problems of it freezing in the trough before the birds could eat it, so I just took to putting it directly on the DL and never had a bit of frozen feed after that. That was really cool.
I'm still marveling over it and I finally feel like this DL is working well...so well, in fact, that I could finally remove well composted material out of the coop after 3 years of accumulation. Three years I've never cleaned it out, but now it's making compost like crazy....I screened out a couple wheelbarrows full this spring and will be doing the same this next week. The larger particles get returned to the coop, the fine composted material will go on the garden.