The typical battery that ships with the Futaba is 4.8V and something around 700mAh. As a thumbrule, the larger the mAh value, the longer the battery charge will last and the faster it will become unusable.

The 4.8v Futaba battery will be good for 3~4 fights like Landeck suggested but I would suggest you get yoursefl a 6V (5cell pack) of atleast 1100mAh rating. As with the 55AX, it will help you build additional safety when you go into aerobatics. At our club we make these ourselves. I just lost a 2200mAh 6V NiMh set that I had used every week for more than a year, but during the week it would almost never discharge and I am into 3D flying with 90-size engined airplanes and hi-torque servos, so that was quite good duty they gave.

If you are on a budget, the Futaba battries are good enough for now

Ameyam