As long as it is sewing okay, I don't change it unless I need a different size.

every 4 - 6 hours of sewing. If you think about it is the cheapest, yet most important part of the sewing machine. Without a nice sharp tip on your needle the quality of your stitches will not be good enough.

After every 8 hours of sewing or after every project whichever comes first. Not changing your needle can cause your timing to go out, resulting in skipped stitches, or what appears to be tension issues thus leading to costly service bills. Needles are cheaper then a service bill.

If I'm just sewing quilt tops, table runners, etc. I change the needle when the stitches start getting wonky. I always put a new needle in when I begin quilting several small pieces and/or a large quilt project.

On my Vikings, I change the needle after every 8-12 hours of sewing or embroidery.

I change it depending on the thread I'm using, but otherwise I wait til it goes "crunch"!

My machine says tick tick tick when the needle gets bad, I've gotten into the habit of waiting for that little noise. Probably hard on my little "Shasta". but I don't waste a lot of needles

That depends upon how big your "project" is. A bent or scarred needle with a burr can lead to wear on your thread and fabric as you piece it...not to mention lots of jams. I try to "listen" to my needle. It makes a certain noise as it goes into the fabric. When it gets dull or gets a burr, I can hear a different noise...a kind of quiet popping as it goes thru the fabric, as though I were sewing thru thin paper. Regardless, I usually change it every 10-15 hours of strong sewing. My machine just runs more smoothly. And if you are buying them at JoAnns with a half off coupon, they are really cheap compared to the hassle of a jam. If I have any kind of thread problem, putting in a new needle is the first thing I do.

When I hear it starting to make noise while penetrating the fabric. That is a different noise than a freshly sharp needle. Definitely at the beginning of any new project if it is a large one. Sometimes in the middle if I have hit something with the needle. If you buy your needles in bulk you will always have one ready to go when you need one. It is a real drag to have to drive across town for a needle.
John

When it breaks or when it begins to "pop" as it goes through the fabric.