For piecing, I generally change it with each new project. I don't do a ton of piecing, though -- definitely way less than 10 hours per quilt. (I've always heard to change every 8 hours -- go figure)
With quilting, I use a thick cotton batting (Warm & Plush) that dictates the necessity of a very sharp needle so I'm changing my needle fairly often with that. Definitely at least every 6 hours, sooner if my stitches aren't forming as nicely or I see it hesitating before pushing through the quilt.