it is *generally* recommended that your needle to changed with each new project- or after 8 hours of sewing time-which ever comes first---very few people actually follow the recommendation- but if you do---you will notice a more consistent (good stitch) when your needle is bent, dull, or gets a *rough spot* it is definitely time to change it; if you are seeing skipped stitches, or uneven stitches---it is time to change; if you notice a change in sound- or find your thread is starting to break, fray or create more *fuzz* it is time to change. the needle should be the correct size/type for the thread you are using and the project you are working on- if you are unsure about that visit a thread company (Superior Threads) is a good place to start- they offer charts you can print off (free) that tells you what needle *size/type* to use for what thread you are using (weight/type) and what each is best suited for- they have a wealth of information, hints, tips, advice - they are a great resource- will teach you a lot about threads and needles.