That depends on you. I wax my string and cables more often than most I guess. At least once a week. I'll wax 'em again before I have to go shoot in wet conditions. Same for dry and dusty conditions.

On the other hand, I know some guys who refuse to wax their strings, ever. Some of them have been known to soak their strings in acetone to get all the wax out of them. They are conviced that string wax adds too much weight to the string and slows down their arrow speed. I think they're being more than a little silly, but it's their business if they want to be forced to replace their strings and cables at least twice as often as I do.