Claytons:
P21S GEPC can be used 4-6 times a year with no problems at all. It's an extremely mild cleanser, but if you maintain your car very well, you shouldn't need to use it more than a few times a year....perhaps when you start to notice any diminishment in shine.
Also, after testing both products for a while, I feel GEPC is superior to IHG in every way. It leaves a markedly better shine, is easier to work with, 'seems' to contain a smaller percentage of solvents, & does a better job removing/filling small surface imperfections than 3M's IHG. It also leaves a slicker surface, completely free of any streaks. In fact, after using GEPC, the shine is so good that after 'sealing' it with P21S carnauba it's not that easy to notice any difference.