Well, you want to try to notice it 100% of the time. While you're doing it though you'll occasionally find that you accidentally quit noticing it. When that happens, just go back to noticing it like you were before (if you were counting instead of doing the pain anchor you'll find you've stopped counting).
Don't beat yourself up about failing to continuously notice it, but realize that that's what you're shooting for and keep going back to it. You don't have to do it perfectly, you just have to keep doing it.