The primary purpose of job costing at our shop is to help us figure out better methods and batch sizes. Sometimes it's just to sell the guys on an idea.
The natural instinct is to regard bigger batches as better batches. What is useful, however, is to see whether or not building ten drawers at one time really generates any savings over building just the drawers you need.
At our shop the machines are already dialed in. We have reduced set up costs to simply turning a switch on or off. As it turns out the per unit time to build one drawer in a batch size of ten is no improvement over building the drawers in a batch size of four. Managing 20 pieces of stock is a lot simpler than managing 50.
We do, however, sometimes cut all the drawers at once. This decision is usually based on material yield.
Understanding how long things take is also a useful clue to evaluating how well someone has mastered drawer construction. After parts have been cut out it usually takes about 10 - 12 minutes to machine and assemble a drawer box.
We get the same time when a veteran does this or when a new person does it. We have the new person record his times per operation on the much maligned laminated SOP sheet. This is mostly to help them dial into the concept of what waste is. The guys like it when they hit the time right, especially the new guys.
We do a lot of flush inset mortised hinges.
Much of the minutes is just putting the door on & off the cabinet while it is being fit to the frame. The on & off part is entry level work. The fitting takes skill. By understanding how these skills distribute we can usually trim some time off this phase of the project. Without this understanding of time the whole project would be done by just the craftsman. A cabinet would take longer to get through this phase and the newbie would spend more time polishing concrete (though they now spend this time practicing fitting doors to a dummy face frame)