Sewing, going through fabric, dulls the needles' point and will eventually start pulling the threads of your fabric instead of piercing through them or going inbetween the threads. It's not a thought the manufactures thought up. I will change mine maybe every 2-3 weeks depending how much sewing I do. If I have made a large quilt, say a queen, I will change it when I finish the quilt. I definately change it if the sound of the needle going through the fabric changes sounds, kind of a thump,thump. Also if I hit pins, that really dulls and can cause burs on the needle as well as damage your fabric. Hopes this helps.