I just bought a Futaba 7CAP. I notice the TX battery and the RX battery are both 9V 600mAh and 4.8V 600mAh respectively. The current drain for the TX is 250mA and the RX is 16mA. Can anyone explain how I can find out approximately how long the battery will last. Is there an easy calculation or do I need to know physics?

Any recommendations on battery upgrade so they may last longer. I was able to find plenty of 1000, 1500, 2000 mAh batteries for the RX but I cannot find any for the TX. Are there any that will fit my TX or am I going to just have to live with it and buy another 9V 600mAh .The battery in the TX is 1.125" x 1.125 x 4".