There is nothing better for a good string than a good silicon based wax. When can you tell if it needs waxed? When you start seeing some fuzzies is one good sign. Another is that if you rub it with your fingers and it feels dry. How often? It's up to you. You can't overdo it. And it depends on how well you take care of equipment.

I shoot every day, but always case my bow when not in use. I probably wax my string only a few times a year. And I replace it every year. The cable should be replaced every three years, but a rarely have a bow that long. In fact, about 1 1/2 years is the average.