When I start hearing that "tick tick tick" dull needle sound, or when it breaks. I make a lot of zippered pouches and purses that have several layers of fabric and interfacing, so my needles usually bend or break before they have the chance to get dull. Speaking of which, I hate the way my machine sounds when a needle bites the dust. It makes a startling mechanical "scream", freezes up (safety mechanism), and beeps frantically at me. Okay, that's a slight exaggeration, but it still scares me every time!
LOL!