How often should I have my figure skating boot laces replaced?

I keep having problems with my laces coming loose on my skating boot, which is a Jackson Classique, causing my heel to slip upwards inside. I have been putting on my boots so that my heel is at the very back of my skate. I've been pulling the laces as tight as (comfortably) possible, but I often find myself having to re-tighten/tie my laces, even after just having the skates on for a few minutes. My laces don't show signs of wear. They just keep coming loose.

That being said, how often should the laces be replaced, and is there a particular blend of materials that works really well? What else can I do to make sure that my laces stay snug?

Thanks!