When it breaks. I used to change it with every quilt, but it seemed so wasteful to me. Now I change it either when it breaks or when I can hear it popping.
I use the Bubble Jet, too. Love it. Feels just like fabric and prints almost as crisp as the original photo. I made a memory quilt for my grandma when she went to the nursing home, so it got washed a lot. Looks just as good as new!