When I can't remember the last time I changed it? I know that's bad, but unless I'm having trouble, I don't think about my needle till I give the machine a drop of oil. My dealer said her rule of thumb for when to add oil is when you think about it. I like to clean out lint from everything--sewing machine, dryer, clothes--so I really should train myself to be more vigilant about changing the needle. I do change it after finishing a big project because I won't remember when it comes to changing it before the next project--unless I remove it completely till I sew again.
With my iphone, I guess it should be easy enough to put a reminder on it. But how often? I would never keep track of every 8 hours.