It depends. For the most part, you can’t eat too many cheerios. I don’t know if you know about the Cheerios Diet? Essentially, you can eat all the Cheerios you want. For dinner, lunch, and breakfast, if you want (so long as you do it in that order).
There’s one thing you should be aware of, though. Ever since the cheerios Moon Rocket Fiasco, it has become a bit of a risk to eat Cheerios under a full moon. It’s kind of a reverse werewolf thingy. You kind of get hungry for a different kind of snack when you eat Cheerios under a full moon, as you can see, if you follow the link. But you knew that, already.
Anyway, as long as you’re careful (you do use condoms, don’t you?), and keep a close eye on the lunar cycle (living with a woman is good for this, but don’t tell her I said so), you can safely consume nothing but Cheerios for the rest of your life.
Not saying how long that might be, but just that it’s possible.