Convert Plugin Christmas decoration to a battery operated light
well I would recommend LED's about 10 for low power usage and bright light how ever for the timer that gets a little more advanced you have four options the first you can hope to tine one online-----
oh one more use the circuit from thos lawn lights use a transistor to use the other battery you might have 
third use a arduino...
forth you could use two 555 timers one in monostable and the other in astable 
the mono stable would turn on a D-TYPE FLIP FLOP and trigger the astable 555 to after the desired amount of time turn off but this requires time to set it up every year...