How to power servos externally in Arduino
Using my favorite search engine I found https://www.servocity.com/html/s9602_hi-speed_mg_bb.html
It's a close match. However frustratingly the page does not contain the current required. So now you have to find the Data Sheet from Futaba in this case ...  Which I could not find either....

So now I drop back on the Plan B and measured the current when loaded!

Take that value and multiply by the number of servo's

So whatever the total value is in Amps and you want it to run for an hour (Say 2A) then you need at least a capacity of 2Ah (Amp hours) at 4.8 to 6 Volts for your battery.

If you only want it to run for 30 minutes divide by 2 etc ...

Will it be exact, Probably not but good enough to get you up and running in the right direction.

Connect the battery negative to the Arduino
The Battery positive to the servo positive only. 
Then the signal wire to the Arduino.

This is the simplist way, don't connect the Arduino positive to the servo battery positive as the servo will introduce a lot of noise and cause more problems (random resets and the like)

You can get round this with filters and regulators but that is more work.