GBC Mod

[N00BModdr]

There's a mod I'm trying to do (among several), and I know little about electronics.
I am trying to put a front light on my GBC. I know that there are many ways to do this, most likely the best using the light from an old frontlit GBASP, but the way I want to do it is with LEDs. (Any help or tips would be greatly appreciated)
My question is: If I used 4 LEDs, by about how much would this affect the play time?

[alentris]

It would'nt affect the gameplay more than a minute or so, if that.

[N00BModdr]

Thanks!