2-3hrs battery life on modern games is really good. That would be considered optimized settings for battery life when it comes to high-end handhelds. You can drain the Deck battery in 60-90min with uncapped frame rate under full load with high-ultra settings.
If you’re willing to play at Switch settings you can do better than V1 and Switch Lite, coming in just under V2 at max brightness + WiFi. For Doom Eternal at 600p/30fps without res scaling at Switch settings, 100% brightness + WiFi with TDP limit at 7-8W you’ll be looking at ~3h45m in mainly combat scenarios. Switch V2 hits ~4h7m, but with dynamic res scaling from 612p max all the way down to 360p min.
Hell, if you’re willing to play at 30fps/768x480 FSR, 55% brightness, Airplane Mode, rumble and haptics off, at 5W TDP limit you can hit ~4h30m battery life in Eternal. The Deck offers a lot of flexibility in that regard. For my tastes, I like settings that will hit around 1.5-2hrs in modern AAA games. My Deck play sessions rarely exceed that.