I'd base it on how large they are, basically if they're about hand size or not. I'm currently eating sardines occasionally. But I was worried recently that I had eaten mackerel for too long. For maybe a year or two, I'd have them multiple times per week. And they're not a small fish, on par with salmon. I was worried I might have mercury overload. That was months ago, so not sure if it would have naturally detoxed by now if that were the case.