Fescues take seven to 15 days to germinate. This may vary slightly, depending on the soil's moisture content and the climate.
Wait until the new fescue is 4 to 5 inches tall to mow. Set the height on your mower to the maximum to avoid cutting off too much. You want to encourage the grass to spread, not strip away the grass' ability to photosynthesize. Make sure the grass is dry before cutting it.
Mow tall fescue at a height of 2 inches in the fall and spring and 3 inches during the summer and if it's planted in shade. Make sure to never remove more than one-third of the leaf blade in each mowing.