I have (as I see many people have) struggled with my smoker a good deal.
Based on what I have learned from that struggle, I am offering this as my perspective on keeping a smoker going for a long time.
Lets begin with the basics (I didn't think of this at first).
In a smoker, you have a fire.
The way fire works is, you have a source of heat which is burning. To do that, it needs oxygen and fuel which it can heat up to the ignition temperature. As long as the unit of fuel that is burning releases more heat into the unheated fuel than is needed to heat another unit of fuel to the ignition temperature, the fire grows larger.
If the fuel is wet, then it is harder to heat, since the water needs to be boiled off first. This is why a fire made with green wood is so difficult to keep going.
In a smoker, the situation is somewhat different, since the design of a smoker provides for a very limited supply of air. Ideally, the fire in a smoker should burn at a rate determined by the available air supply. What that means is, when your smoker is just sitting there, the amount of fire is limited to the amount of air that drifts through the smoker driven by the convection currents the smoker creates.
If that amount of air isn't enough to heat enough fuel to the combustion temperature to keep the smoker going, the smoker will go out.
So, it helps a good deal to have fuel which heats easily (is very dry).
I mostly use hardwood pellets, as they are not much trouble. However, I live in southeast Wisconsin, which is not the driest part of the country.
TO my surprise, I discovered that if I use wood pellets kept in my shed, indoors, but not heated or air conditioned, they burn about 20 minutes. If I use pellets kept in my garage, which has a dehumidifier, the smoker will burn until it is out of fuel.
In both cases, the pellets are "dry", but in the shed, the relative humidity averages 80% (lower during the day, higher at night), while in the garage, the relative humidity averages 50%.
Wood and similar materials very quickly absorb moisture from and lose moisture to the air as the relative humidity changes.
I have observed this multiple times. The pellets are from the same bag.
So the main thing to keep a smoker going until it uses all of the fuel is to have truly dry fuel. If you live in Arizona, this might not be much of a problem. But in most of the USA, getting the smoker fuel really dry will help a lot. Keeping it in an air conditioned or dehumidified area will help greatly.
The second conclusion is that taller smokers will burn better/longer. They have a taller chimney, and will draw more air. more air means more heat, which vaporizes more fuel, etc. If I lay my smoker on its side, it goes out pretty quickly, because the air intake and exit are at the same level, and it can't get any air. (Provided the lid seal is tight)
Alas, my smoker is pretty short. I looked at those tall smokers, and thought they looked funny. Now not so much.
As regards fuel, most all plant based fuel sources have about the same heat value per pound - a pound of pine or a pound of hickory or a pound of cotton all produce about the same amount of heat when burned. So if your smoker is burning well, the way to make it burn longer is to get as much mass of the fuel of your choice into it as possible. Packing the fuel may also reduce the air flow - depending on your smoker design this could help it burn longer, or possibly make it go out...
Other than that, use whatever fuel you want. I am very much an amateur when it comes to smokers, and am very happy if I can get my hardwood pellets to burn nicely and produce smoke for 3 or 4 hours reliably. When they are dry, the burn until they are nothing but ashes, which takes quite a while. I don't know how long, since this only happens when I forget my smoker on, and come back the next day...
I'm writing all of this because it took me a surprising amount of time to figure it out, and most of the posts I have seen on the topic (not an exhaustive search) have focused on what fuel to use and how to light it (I use a torch), and not on the relatively simple chemistry and physics of fire.
I hope this will help people who have struggled with this.