Battery life will depend on just how many LED you put in there and what you drive them with and what you drive them through.
If you use 1 led and limit to 30mA or 20 , plan on a wild increase in battery life.
If you use primay cells, the life will probably be close to the shelf life of the batteries since you're probably not going to use this 24/7. If you use rechargable NIMH or NICAD, they self discharge and you will probably be able to use them longer than you eventually feel comfortable with since you have to be careful not to discharge the rechargables too far.
Remember too, that most rechargables drive @ 1.2vcd for most of their discharge cycle while primary cells push 1.5 to 1.7 and dwindle to nothing.
A circuit designed for rechargables might allow dammage to the LEDs if some new high performance primary cells are used.
As far as the "through" part, you'll need a regulator circuit be it a constant current source or small light bulb(resistance changes with temp.)or simply a resistor.
Pack in lots of LED's and battery life is reduced but the light can really get bright still with much less power than a regular bulb.
If you want to make your own and battery life is top prioity, keep the drive current down on the LED to put them in a more efficient operating range. It'll cost pretty good to add lots of LEDs though.
My white "night light" runs for 1/2 year on two aa cells and it runs 24/7.(Some white LED will run on <3vdc)
Maglites make nice LED flashlights!
interesting stuff.