Once the knives are put on, they kill you in about a week. You can only live up to a month if you take the rings off It was explained in The Gathering Storm, Ch. 36 The Death of Tuon

"Fortuona moved to the next soldier in the line of five... These five were Bloodknives. The pure black stone ring each one wore was a specialized ter'angreal that would grant them strength and speed, and would shroud them in darkness, allowing them to blend into shadows.

The incredible abilities came at a cost, however, for the rings leeched life from their hosts, killing them in a matter of days. Removing the ring would slow that process slightly, but once activated - done by touching a drop of one's own blood to the stone ring while wearing it - the process was irreversible."

So you see, they are good for nothing except suicide missions. No loopholes.