You’re certain that you’ll be operating each and every motor at its maximum load, such that ALL 15 servos draw 5A each? Normally in a humanoid, the current draw is very different for different joints. If you use 3x 6Ah = 18Ah batteries, and you determine that you’re actually drawing 75A, it’ll last 18/75 = 0.24hs = ~14 minutes. This does not factor in inefficiencies of the battery, controller, motor etc. which can easily reduce it by 50%. You should take a look at each motor and see if you’ll actually be drawing a full 5A. You might need to calculate or estimate the torque at each joint under normal operation