The short answer
For many clients, a fade looks its sharpest when refreshed every two to four weeks. The right interval depends on how quickly your hair grows, how high the fade sits and how crisp you like the transition around the sides and neckline.
Fade height changes the timing
A high or skin fade creates stronger contrast, so new growth becomes noticeable sooner. A low fade usually grows out more softly and can remain balanced for longer. If you prefer a consistently fresh, close finish, shorter intervals make sense.
Your haircut above the fade matters too
The sides are only one part of the cut. Length and texture on top can change how long the overall style feels intentional. A good maintenance schedule considers the complete shape rather than refreshing the fade in isolation.
A useful routine
Instead of following a fixed rule, notice when the outline, temple area and transition stop looking the way you prefer. Tell your barber how long it has been since your last cut. Over a few appointments, you can establish a schedule that fits your growth and your standard of sharpness.