I have to watch myself as I'm prone to pushing down too hard and that dulls the blade faster. I change my blade when they skip so bad I start cussing, then always wonder after I change it why did I wait so long?
As for changing the needle in my machine, I do it when I hear that it needs changed. That distinctive "puck-puck-puck" of a dull needle as it enters the fabric tells me it's past time to change it. Sometimes though when I'm starting a new project and can't remember the last time I changed it I'll do it as I'm cleaning/oiling the machine to ready it for the new project.
Cari