There's a couple of things.
A fire normally reaches it's maximum heat 2-3 hours after lighting, from then on it's all down hill. So, a 6 hour burn your fire may be hottest 2 hours after lighting, and for the next 4 hours it goes down from there.
Next, the water in the wood has to evaporate before the wood can burn, the less water you have in your wood the less time is needed before it burns and less heat wasted on evaporating water instead of burning wood. I've had a situation where the wood was from the bottom of my wood pile and wet. It took me 2 hours to get a dinky flame, and for the next 4 hours it didn't get bigger and produced nearly nothing for heat. All the heat instead was used to evaporate water. I burned wood stored in my basement a year and it nearly instantly took flame and produced tons & tons of heat. Dry wood is a pleasure and easy to use, wet wood is difficult, problematic, and not enjoyable. If your wood is very wet, even with full air you'll find it smolders the entire time and you wish you could give it more air. I've found wet wood doesn't burn very long. The entire time I'm at full air, the wood smolders and after 4 hours there isn't much left and I sit back and say where was the heat. Whereas, dry wood I have to turn the air down and I can get a long burn time and a lot of heat.
The type of wood makes a difference, all wood generally has the same heat per lb. The difference is how dense your wood, like oak is more dense than birch. Birch being lighter, dries faster, burns faster, and hotter whereas oak being more dense takes longer to dry out, is more difficult to get started, and takes longer to evaporate the water out of it but burns longer. A trick is to always put your driest, smallest logs on the bottom and your wetter, larger logs on the top. If you don't have much in terms of small logs, put your lighter wood on the bottom like your birches and your heavier dense logs on top of them like your oaks. That way your kindling has a chance to evaporate the water in the smaller/lighter ones they will ignite, as they burn they will hopefully evaporate the water in the larger dense logs on top and they will ignite and you'll get good heat. The opposite of putting your wetter, larger logs on the bottom and smaller logs on top is that your kindlin will burn out before it's had a chance to evaporate all that water in the larger logs, and they sort of smolder and don't produce much heat for pretty much the entire burn. Flames, you want flames.