If only there were a magic formula to figure out how many 1.5 volt batteries it would take to equal 9 volts.

Oh! There is! 9 divided by 1.5 equals 6! Take that, Fermat! I'll take that Nobel Prize now, thank you.

Now, as to how long the batteries will last, you'll have to google for that, dividing the total mAh of the batteries by the load of the circuit.