I change mine about every 4 to 5 sewing hours - its hard to gauge though, you might have been 'sewing' for 6 hours the machine might have done half that, what with cutting out and pressing and rumaging etc....
I will look at the Titanium needles after my current supply runs out - thanks for the tip.
BTW - I never wait until the needle plunks - not very good for the machine and I also hate the needle to decide to be blunt right in the middle of something special.
When I used to buy needles a 5 pack at a time it was hard to keep tossing them, but then I started to buy the 'workshop packs' from my Bernina dealer - 100 needles in a little box, and its not so hard to throw one out now - the boxes still last me ages, I have about five different boxes on the go for different jobs, so thats a lot of needles!