I am in the "when it is needed" camp. I try not to wait until it breaks, but I can usually hear and see the difference in the stitch between and sharp and dull needle. I am also in the camp of "run over the needles" which also limits their life (lol - despite the fact that I slow down there is an occasional nick). I usually I have several projects going on at once .... so "after I finish a project" never works for me.