One cannot be too open-minded, this is like saying one can breath too much.

“Challenge conventional wisdom and never accept the truth of something merely because everyone else views it as obvious.” Fredrich Haller (Einstein’s first employer and his advice to him.)

You know you have reached the point of too much open mindedness when your brain falls out of your head.

When you accept everything, and not question it.

One can never be too open minded. Questioning everything is a very good way to go through life; never take anything in life for granted.

How about questioning gravity? That sounds like a step toward insanity.

When, after considering the facts of a particular subject, you are incapable of retaining the opinion which you formulated and are able to be swayed by the views of others then that is open mindedness working against you. You will be perceived by others as someone of weak character whose opinion doesn't matter.

Being extremely skeptical, while perhaps not insane, would impair one's ability to accomplish anything. For instance, how do you know the floor is going to be there when you get out of bed in the morning? Even if you can see it, and it looks solid, and it's been solid for your entire life... It doesn't mean it will be solid this time.

The question, I think, is not how much you can question, but how well you could handle your own skepticism.

We're at the point at which we don't even know if reality is a thing. At this point, there's nothing you can't question. However, what conclusions you come to are what will affect your attitude. If I see a paper move in a windless place, I could just say that I have no idea what did that. If I make that conclusion but can't handle it, I might feel confused, anxious, and maybe even a little paranoid because of how much I'm questioning reality. Or I could just shrug it off.

Question reality all you want, just try and live with what you have whether you assume it's real or not. Kinda like God: the universe seems to be the same with or without it, so why stress about knowing?

See the definition of 'sheep' .