I too use the titanium needles and love them! I change mine when I seem to notice a different sound coming from my Machine. Although it is suggested to change the needle after every project...but I find some project are not that big so I go on sound.
I just recently was sewing on a button and the :evil: button hole wasn't right. My titanium needle when right through the button didn't break the needle, matter of fact it is still in the button. Any other needle would have broken the minute it hit the button.