But over a longer time period, Flo can build a new larger chicken coop if she wants to produce more chicken. Different inputs can be altered at different times. For example, if the number of workers can be altered daily, and the number of machines altered yearly, and the size of the plant every 10 years. Then 10 years is the long run. All costs depend on the firm's planning horizon.
All costs are variable in the long run.
In the SR, Flo can only chose how many chickens to squeeze into her chicken coop. But in the LR, she can chose among different coop sizes. Consider the diagram of AC below. The long run AC curve shows the lowest possible short run AC corresponding to each output level.