My 3.5 year old iPad Air has a wide range of battery life depending on the game and the brightness settings. I chose "Real Racing 3" as "worst case scenario", since this racing game has detailed graphics and is rendered in the native resolution of 2048 * 1536. With brightness to the max (which is much brighter than any of my other devices) and Wifi on I managed to drain the battery in 4 hours. With half brightness (similar to full brightness on other devices) 6 hours are possible, up to 8 hours with low brightness + flight mode.
The iOS Version of "GTA Vice City" can run between 5 and 11 hours, "Ace Attorney 3 HD" as "best case scenario" 5 to 15 hours:
My 1 year old smartphone can play demanding games with half brightness and all connections on (cellular, LTE, WiFi and a BlueTooth-connected DualShock4) around 4 hours, no matter if it is a native Android game (GTA Vice City), an emulated PSP-game (GTA Vice City Stories) or an emulated PS1-game (Crash Bandicoot 3).
In flight mode probably more, but that's no viable solution for me since I don't want to miss calls or messages by putting my phone in flight mode for many hours.