Short Answer: When it no longer makes sense for the conflict at large.
Long Answer: Creating characters requires some understanding of the plot, and creating the plot requires some understanding of the types of characters that will fulfill that plot's conflict. Setting has some influence in this as well, but the biggest influencer is the conflict pertaining the plot.
I'll explain a few things first, I guess, then make my point.
A conflict is the overarching issue that pushes a story forward and motivates characters. Conflicts can be anything--personal or impersonal, grand scale or local. They can be about stopping the evil emperor from conquering the galaxy to whether or not Gary and Mary are gonna wind up in a relationship or not. Stories often have multiple conflicts going on at once, and the gradual progression and resolution of those conflicts is what produces a plot--which is a series of events that occur in a traceable, chronological order.
Characters are the motivators of change in a plot. They're the ones that triumph or fail in their attempts to achieve their individual goals. The series of events that occur in a plot are based entirely on the characters and what they do. Therefore, a character's characteristics--their history, their personality, their skillsets, so on--need to reflect something that can allow them to have individual power to influence the conflict, and motive to want to see it resolved to some particular goal or end that they have in mind.
What combines in all of this--the characters, the plot, the conflict, so forth--is what ultimately produces emotional satisfaction or intellectual enlightenment, which is the two core points on any piece of fiction--to enlighten, or to emotionally satisfy. The struggle a character has in achieving their goals, or success or failure of those goals, or the mutation of those goals over time through interactions with other characters and interactions with the conflict, is what sparks life into a story.
Thus, to answer, in more detail: A character's personality is "too much" when the character itself can no longer be justifiable as a protagonist that is both willing and able to cause noticeable change in the conflict of a story. If a personality trait makes a character too incompetent, or too extreme, that is a problem. If a personality trait makes a character unwilling or unable, that is also a problem.