I change needles once a year or when there is a problem. Many people would say that's not nearly enough. One well-known professional said they change needles weekly. It depends somewhat on what you're embroidering. If you embroider a lot of heavy items like bags then your needles will receive more wear. If you embroider fine items like lace handkerchiefs then you'll want to change your needles more frequently. If you embroider a lot of fine text using a fresh needle may slightly improve the quality.
I had this same question a few years back and got lots of answers. I have a magnifying glass with one of those little SUPER magnifier smaller glasses in the handle and in good light you can see the tip very clearly. If they start making more noise than I think is normal I will pull it and check and most times the very tip is starting to deform so I toss it. I change needles fairly often because they just don't cost that much but since figuring out what they look like when worn I throw the used but still good ones in a special spot and occasionally grab one if I am running low.
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