When diminishing marginal returns set in total product?

Diminishing marginal returns occur when adding an additional unit of a variable input, while keeping other inputs constant, results in a smaller increase in total product. This typically happens after a certain level of input has been reached, where the optimal combination of inputs is exceeded, leading to inefficiencies. As more of the variable input is added, the incremental output produced declines, highlighting the limits of production capacity in the short run.