My problem isn't so much not changing often enough, or not changing when doing something completely different (ie paper piecing vs piecing batiks), my problem is that I sew so infrequently and don't necessarily finish project A and launch into project B in the same day - so by the time I go back into the sewing room I forget what needle is in the machine. Some (all?) needles have markings on the shank that tell you what they are, but my husband took my mag glass to his shop so I'll be darned if I can see what the markings are! So I don't know where to put it back into my needle collection! I need a good "habit" ...perhaps taping the needle case to the top of the machine so I know which needle is in there or something like that. Of course that will only help when I pull a needle from a case of all like needles - I think I still have a pkg of "mixed" that is my main problem. I won't be buying mixed size pkgs anymore.
As for changing needles ... If I keep the same needle in the machine from start to finish through a project I'll change it at the end of the project. Otherwise I'll leave it in until I have a failure of some kind (ie the pop or thwack it makes when it hits the fabric, or thread breakage not caused by other means).
Currently I use Schmetz and I have some Klasse. Might give those organ titanium's a try the next time I need needles. I just bought 7 pkgs various sizes/types (Schmetz) on an amazing sale (70% off with my completed thread card and the purchase of 10 additional spools of thread!!).