I can be a terrible bore when talking about my chronic allergies or compound hyphenation, but I am hypersensitive to the second I see anyone's eyes drift or glaze over. And who knows how long that takes! But the second I do, I try to steer the conversation to more interesting waters, like your allergies.
Whether it's body language or canned responses, I'm sure you all have grace-saving tactics to recognize when you've said enough. How do you know when it's time to change it up?