I've never been able to figure out how to keep track of how many HOURS I've acturally been sewing. It's hard enough to make estimations of how long I've been in my quilting room putting together pieces, trying colors together, digging for just the right material/template/thread, staring at my quilt wall, sticking fabrics up next to others... nope, too bright, try another..... etc., etc., etc.
So, like most people I wait until I hear the pop or realize things aren't going as smoothly as usual. I change needles from piecing to quilting, or if I'm using some heavy fabric, but I put those "slightly" used needles back into their proper case upside down, and use them again later when I have their special need.