DC Motor Power Supply Connection (Arduino)
As you can see from http://en.wikipedia.org/wiki/Nine-volt_battery:

9V batteries, generally, has a very low capacity. Motors, usually, has a high current drawn. So all you have to do is measure it so divides the capacity of your battery by the current drawn you have measured from motor to find its autonomy in hours.

If you want to specify for how long you want your system working just multiply your current drawn for how many hours you specified. So you will find a value in mAh. Divides it by the typical capacity of your battery and then you will realize how many batteries in parallel you will need.

In your case, I hope you have not fried your Arduino which has a 50mA current draw limit. Supposing you have an alkaline one uses a 1N746 in series with your motors. Take a look at the currents draw of your motors first to use correctly your zenner and battery.