Depends on the game.
I'm currently playing Morrowind and I get like 6 or 8 hours, which is crazy for a portable device imo. I'm used to a PS Vita and a first batch Switch, both having battery lifes of what, 3 hours?
Then you have AAA games on the Deck. And yeah, games like Cyberpunk 2077 and RDR2 might drink the battery in about 2-2.5 hours but man, if you ask me, being able to play those kinda games on a handheld is great, even if for just a few hours.
But as others said, the Deck also gives you tools to make it last more time. Reduce the brightness, turn off the wifi, lower the screen's refresh rate, turn down TDP or even download PowerTools and turn off some CPU cores or put your RAM in power saving mode. No other handheld has this ammount of customizing options, afaik at least.