I use different needles based on the project I'm doing, techniques I'm using, fabric/thread requirements. Because of the nature of my projects, I often use a different needle for different parts of a project--I might use one needle for piecing a top, another kind of needle for doing any decorative embellishment if the thread calls for it and yet another kind of needle for FMQ and binding. So it really doesn't make sense to me to start with a new needle for each new project. When I switch one needle out I put it back in the needle case it came from for re-use later so it's kind of hard to figure out how many hours I've been sewing with a particular needle. I usually do a small test piece before starting to sew, so have an idea from that if the needle is still in good shape.
Otherwise, I listen to the sound the needle makes as it pierces the fabric. That's usually a pretty good indicator if it's time to change the needle out--the sound changes.
If your needle starts pulling up on the fabric with a popping sound, or you start getting thread fraying, or your stitch quality deteriorates-- those can be signs the needle has gotten a bur on it or is dull--or it might have to do with lint in your bobbin case, wrong upper threading, etc. LOL.
Rob