When to walk away?

In 1998, the best president we ever had was impeached due to some stupid moves on his part.  To take it one step further, the more he backed himself into a corner in his fear to protect himself, the less sense he made.  An infamous quote by said president was, "It depends upon what the meaning of the word 'is' is"?

Oh dear, question of the day.  How far do you fight stupidity before you've stooped to their level?  At what point do you look at the conversation, no matter how unemotional and/or logical it is, and just say, "Dear Lord, why am I having this conversation?"

It's easy to be caught up in someone else's drama.  With any relationship, regardless of the technicalities, we bring emotion to situations.  This emotion exacerbates the situation in one direction or another.  When should we step back and realize that perhaps we've lost the fight?  We, for whatever reason, can't be trusted to open our own mouth.  When to walk away.