the machine manual for my machine says needles should be changed either with every new project or after 8 hours of sewing- which ever comes first.
i change mine maybe not quite that often but often enough-
if you do not change your needle you (can) start having all kinds of little problems- that go away with a new needle

Please don't use titanium needles. Sometimes when a needle breaks its because something is wron with the machine.. bobbin threaded wrong or in wrong, top thread not pulled through tension disks or threaded wron, a bent shaft, etc. If you use the titanium needle that doesn't break, you run the risk of doing much greater damage to your machine. The needle breaking can also be a safety feature on your machine's internal workings.

My repair guy told me to change my needle when I hear the needle make a soft "pup" sound as it stitches. He says that is when the needle is getting dull.

Since I found http://www.discountembroiderysupply.com/ and can buy needles reasonably priced I change mine pretty regular. I generally use the quilting needles for piecing and the topstitching for quilting. I use the 80/12 the most for piecing and the 90/14 or bigger for quilting since I use a heavier thread. For raw edge applique with invisible thread I use the 60/8s. They're fairly fine and don't seem to fray the edge as bad.

I'm trying to use up most of the universal needles I have so I can use all quilting or topstitch needles.

I'm supposed to change the needles?????? LOL! I know I need to do it more often, too. I'm glad you asked this question so I maybe will think about it more often.

I change needle sizes quite frequently, now how often I discard fabric is a whole different story. I am guilty of keeping my needles way too long, but I do the same with my rotary blades so I am at least consistant!

I change needles when they break, are causing skipped stitches or the thread keeps breaking because of a burr on the tip. I save all of the stubs and ones that don't sew very well in a bar of soap and use them as nails to hang pictures.

I started using the Organ Titanium coated needles and they stay sharp, outlasting a package of the regular needles. I won't use any other now. I get them here:

http://www.superiorthreads.com/shop/...s/description/

It should be more often, but definitely when it starts the popping sound...
I buy the Organ needles as well.
Usually use a topstitch needle, which is the same as a metalfil needle. I use 100/16 for the bowls that I make such as in my avatar, 90/14 for the quilting,60/8 for heirloom type stuff.

I change them frequently. Usually after a big project such as quilting a quilt etc.