There is no single right answer, but there is a right answer for your haircut. The shorter and more contrasted the cut, the sooner it stops looking intentional.
A rough guide
Skin fade: every 1 to 2 weeks. Taper fade or short back and sides: every 3 to 4 weeks. Medium length with scissor work on top: every 4 to 6 weeks. Longer hair you are growing out: every 8 to 10 weeks, mostly to keep the ends and neckline tidy.
Signs it is time
Your hair starts sitting differently in the morning. The sides push out instead of lying flat. You find yourself using more product to get the same result. Any of those means the shape has grown past its window.
Booking ahead helps
The simplest way to stay on schedule is to book the next appointment before you leave, or to book online for the same day and time each cycle. Evening slots on Thursday and Friday and Sunday afternoons fill up first.