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That will depend on what wood you are using. How well it is seasoned and what size it is cut. As well as outside temperature and wind to name a few of the variables. You also have to keep the door closed. Then their is the matter of how much meat did you just add.
It would help if you would add your location to your profile, That way we know your weather and altitude.
Oh, I'm in Virginia. Summers in the upper 80s to low 90s. Winters lower 20s to upper teens.
I'm not real sure about wood size. I can get common smoking woods in chunks and pieces about 1" x 4" if that it's suitable. I'm sure I can get hickory splits locally, I'm not sure about other woods.
I have a 36" reverse flow smoker that was custom-built. It isn't a Lang but is similar in size and operation.
I use splits that are about 6-8" in length and maybe 2-3" in diameter. I've found that, for my pit, I can keep a small, hot fire a lot better with smaller splits that combust faster. I add a new split about every 20-30 minutes. If I add two of these splits at once and shut the dampers down more, I can keep a good burn for about 45 minutes. But sometimes that will make the fire get too hot even with the dampers shut down simply because it is just too much fuel... then I'll have to crack open the firebox door to let some of the heat out to bring the CC down...
I'm not ready to take on that level of babysitting right now. I have a propane smoker right now. I have to add wood every 30-60 minutes for smoke, but even if I don't it will still maintain the temperature.
36 stretch. I have to add a couple splits of oak every 30 minutes. Eats wood ......if you are doing a night cook, like I am right now, if you don't, you lose temp and takes awhile to get it back up to temp.....but its 25 degrees outside too and calm. Trying to catch a few zzzs leads to trouble if you don't monitor.