I think you just learn from experience as you go, what works for you. If you have a leak, you've waited too long! Make sure you look at your old flange when you remove it. I change every six days, and have several times gone for seven, so I know that I can, but I can see from how much the output has affected the flange, that it was pretty much at its limit. I choose to change on day six to be on the safe side. I have never experienced smell, no matter how long I've gone, but that would certainly be an indication that it needs to be changed.
Terry