Working longer hours often leads to diminishing returns.
diminishing returns
idiomA condition in which additional inputs into an organization, project or process produce progressively fewer or lower-quality additional outputs, and may, in extreme cases, cause the total quantity or quality of outputs to decrease.
We studied for six hours, but eventually hit a point of diminishing returns where we stopped learning anything new.
The company increased its advertising budget significantly, yet the campaign suffered from diminishing returns as the market became saturated and consumer interest plateaued.