A side fire box is where you keep your heat for indirect cooking. Indirect means that the coals/fire and food never meet. You simply transfer heat from the place where the fire is over to the place where the meat is. For an offset fire box, you build your fire with coals, lump or whole wood, and allow the heat to drift over to the cooking chamber. Kind of like an oven with wood smoke.
The larger your fire, the more heat that drifts to the cooking chamber. Using your thermometer on the cooking chamber’s dome/lid, you want to keep the heat in the cooking chamber to about 200 to 235. Watch the thermometer and monitor your firebox, but don’t open the cooking chamber too often (except to baste or mop every half to full hour). Keep adding fuel (coal, lump, or whole wood) to the fire box to keep the food chamber up to temp at 200 to 235. If needed for long cook times, excavate ash to ensure the air vents are open in your fire box and the fire can breathe (gets air flow from the vents….don’t let the ash clog the air intake vents).
With that temp range, you run anywhere from 4 to 12 or more hours (you will have to continually add more fuel!) depending on your cut of meat. Ribs are typically 4+ hours. Butts are 8+ hours and briskets are 10-12+ hours. These are just approximations of course and real time depends on your cut and internal temp of your meat.
But, keep good air flow in your fire box and keep the fuel loaded to maintain the temp in the cook chamber. No need at all to ever put fuel in the cook chamber unless you are grilling directly….steaks, hamburgers, dogs, etc.