If in fact, the original possessor is defeated (killed) by the Fur Nobili before the power can be returned, then the ability stays with the thief...but if a new power is permanently won, the thief spends about 3 days to a week adjusting itself to it. (The time range depending on the strength of the Fur Nobili.) This process is extremely painful due to the fact that the vampire's genes are tampered with inside and out, which often results in black eye color and other such "dark" appearances...
Hope that cleared things up, Gabby.
Good thing you weren't there to see me the first day.
Hold on one second.
(Queenie slips out of the door with Nagini to get something from her room to show Chaos.)