Well, we switch Cabeza occasionally. Not monthly, but occasionally. We used to have him on Wysong. It was OK, but even Dr. Wysong recommends rotating for variety and completeness. So, I would switch the Wysong varieties. I also was throwing in some Nature's Variety Raw Instinct every other month or so.
Then, we put him on Evo exclusively. For some reason, no matter how little I fed him, he was getting fat. I put him on Innova large-breed and he's been keeping his weight nice and trim. I'm not sure what the deal is, as Dr. Holly has informed me that Innova large-breed his higher in calories per cup than Evo, but it's worked. I don't know. Maybe he digests it slower.
Anyway, I digress. We haven't switched him in awhile, but he does get lots of add-ins. He gets meats and yogurt and peas and stuff like that.