Here are the "rules":
1. "Try" to use one blade for the entire month of February. That means if you want to try a "mortal" blade, like a Derby or a Dorco, and you can get it to last 15 days, feel free to use a new Derby Day 16 and continue. (See Rule 3 and Rule 4.)
I do believe that I'm seeing the end of life with my Feather Professional. I've used it 15 days now in my Mongoose SE. I'm now getting unacceptable razor burn, nicks, weepers and not as good a shave as I'd like. I have taken great strides with prep and technique to try to get the best shave I can get. But this is about it I think. I could probably go the rest of the month without issue, but I do not see my shaves being any good at this point.
I think I'm going to load my Mongoose with a Feather Pro Guard and see if I can get that one to go for the rest of the month (or maybe longer). This has definitely been an eye opener for me.