Just got my Janome 4800 back from the shop - I bought it in 2007 & was supposed to take it in for a checkup (free) a year later ... no I didn't - It kept jamming as I was trying to hem a pair of jogging pants. Got a huge lecture when I dropped it off and again when I picked it up to change the needle a lot more often than I had been doing. Repairman pointed out an article about it, cashier pointed out the same article ... the funny thing is I could have sworn that was the second thing I did after I rethreaded the machine when it started giving me problems.
I was told the machine needles are now manufactured out of much softer metal that in the old days.
I'll be changing needles a lot more often from now on ... needles are cheap!

I wouldn't drive my car with flat tyres or needing air. Nor would I drive it if the walls were damaged or showing signs of wear...

I am like Bearisgray -- I change my needles when the machine does not sound right or when I am sewing on a different fabric. I use the titanium needles for piecing so they last a long time -- I have never counted the hours on a needle -- I have a hard enough time remembering what size/type needle is in the machine (but that's another thread).

Same here. Love these needles. Will replace when I hear a little popping sound or if I do something stupid like try to to a decorative stitch with my 1/4 foot on and it breaks!

I change needles when I change the top thread.

From a hand appliqué'/quilter's perspective, I change needles when the thread starts fraying or breaking or shows signs of weakness, and also when it is so bent I can't maneuver it into the correct spot!

I change my needle when the old one tells me it's time to retire it.

with each new project.

Whenever they break or skip stitches.

It's already too late when you hear the needle making the popping sound. Needles cost more than a dollar each but the quality of one's work is woth it.