I never intentionally tested its life of the battery off the base, but having made a few calls and received some most days they have lasted easily over 3 days. If the phone is not being use or not receiving any calls I could see it lasting easily twice the amount of time.
As for a busy day talking on the phone for say 3+ hours, for which I have had a couple of days like that the phone has lasted left off the base until the next day.
I would question why 8-11 days for a phone to be left off base without ringing, I cant see how anyone would not use or expect one call in that period unless they are on vacation so that test results seems pointless. Even if you go on vacation and leave the phone off base for 2 weeks the main phone will still work as that will be plugged into the power socket and will be taking voice messages.