I change my needle when it makes a 'thunk' noise, or when I am changing to a project that requires a differnt size or a thread that likes a certain needle. I just bought some "Inspira" brand that Viking recommends, and I won't do that again. They dull in about three or four bobbins worth of sewing, when I use the Schmetz will last for about a dozen bobbins. I don't like the singer brand needles any more, they used to be great, but now they seem a tad short and can cause missed stiches even in joining just two layers of cloth together. So yeah, about every dozen bobbins, or if my machine requests it, sooner. I have found that the 'organic' cottons (I had some from Birch and some from Jaycyn) that I just worked with dull the needles more quickly, too.