When does ditzy go from cute/sexy to annoying & thinking she's stupid?

When a girl is acting ditsy or playing dumb with you/around you, when does it go from being cute or sexy or annoying and "boy, she's dumb"?

What if she's your girlfriend, when does it go from being cute/sexy/hot/attractive to being annoying and wanting to tell her to shut up?
When does ditzy go from cute/sexy to annoying & thinking she's stupid?
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