if the color of your base is black, then it'll grey out as the wax is worn off. after that use your judgment to decide when to wax it; generally, you don't want it to turn white.
if it's not black, it becomes a smidge trickier, but you really just need to hold it at an angle to some light and see the color fading.
On my board, at least, whenever the edges are showing signs of dryness it's pretty evident even though the base is green. It gets quite white compared to the other parts of the board. I ride in crap conditions so I normally have dry edges that need waxing by the end of every day.