Depends on your game, settings, screen brightness, etc. It's highly tunable with stuff like FPS limiters and even custom TDP limit settings built into the UI.
Just running a Gameboy Color emulator, for instance, you might get 6+ hours. Running Baldur's Gate 3 at reasonably optimized settings from an online guide, you'd get about 1.5-2 hours
You can literally make a steam deck last 30 mins at minimum. Lol. I would say most users are getting 2 to 3 hours with reasonable settings. So many variables at play. Your best bet is to go in with realistic expectations.
Cheaper doesn't mean better. There's also more expensive, which again doesn't mean better. There are no other amd64 handhelds with a working sleep function that I know of.