That's kind of a good idea, but not really a realistic one.
No one care about products that you got paid to promo those.
Readers only want to know about the comics they're following, or somethings that related to the creators.
So if you're putting ad in your comic just because you get paid to so, they won't like it.
There's no rule, because it's up to you to put anything into your comic's episodes, as long as it's legal.