With my OG Droid, after I upgraded, I put it in airplane mode and turned on wifi. I turned off anything that would sync, and I could get 4 - 5 days out of it prior to hitting 15% when the warning pops up. The battery in that device is much smaller than the RAZR MAXX. Also that battery is the original battery so it doesn't hold a charge as well as it used to.
If you completely disabled everything possible on the RAZR MAXX, it should be able to last quite some time theoretically. I would bet about 10 days would be fairly possible. 15.8 days does seem like a stretch, and 21 hours of talk time seems a little high to me as well. I am sure they achieved both somehow though. Perhaps with super ideal situations.