buying advice shift registers

ok, and another question... I don't want to spam the board, so I'll ask here:
Is there an easy way to calculate the approximate time arduino runs on battery?

I'll use around 50 leds. I remember there was some kind of a formula. I should
be able to calculate this if I know what kind of battery I'll use and how much the
LEDs need, right?

sorry for the newb question...

best
yves