How can you tell when it's time to replace a needle?

you should change your needle after each project or every 10-14 hours of sewing. That's what my dealer told me.

I'll probably be in the minority here but I only replace mine when I break or bend a needle. I sew over pins all the time and still, amazingly, break few. That said, I only piece on my machine - no MQ'ing so I can't say if that makes a difference. As long as my machine isn't complaining and the sewing is going along fine I don't bother to change.

I've been told every quilt deserves a new needle. I change mine quite often and on my Bernina I can tell it needs changing when it starts skipping stitches.

I usaully lose mine often so i guess you could call that "changing the needle" lol

The needle will also start making a slight "pop" noise when it pierces the fabric. I change mine after every project sometimes before, when I start to hear the magic sound.

This is what I do -- though I may try to sharpen it before saying good bye for good.

I change my needle only when I have a problem or it breaks. Sewing machine dealers are like any other saleperson - sell sell sell! If it works I use it.

I talked to a professional seamstress here and she does not change after each project. If it drags or pulls on fabric or breaks then she changes. I do not change my needle as often as I have seen others do here and have had no problems. Personal choice I think.

I change mine if it shreds the thread or causes other issues