I'm in a situation where my iPad Air 4 is only 3 weeks old and I can exchange it for an Air 5, and the only difference between the two that would matter to me is battery life. I decided to stick with my Air 4 because it's battery is definitely better. Some datapoints:
The new iPad Air packs M1 power into a thin and light tablet
www.tomsguide.com
"On the Tom's Guide Battery Test, which involves surfing the web over Wi-Fi at 150 nits of screen brightness, the new iPad Air lasted 10 hours and 9 minutes. That's excellent endurance and a bit longer than Apple's claimed 10 hours.
The previous iPad Air 4 lasted a slightly longer 10 hours and 29 minutes."
So that's 3% better battery life, an extra 20 minutes.
Engadget reviews Apple's 2022 iPad Air, which features the company's M1 chip, 5G and a new front-facing camera.
www.engadget.com
"....but the Air 5 lasted a long time while watching movies.
I looped a downloaded movie in the Apple TV app for four hours and the battery only dropped to 75 percent."
So I was really happy to have found this, last night I downloaded a movie on Apple TV app, charged up to 100%, and ran it for exactly 4 hours and the iPad Air 4 dropped to 83%. So that's a betterment of 8%.
Lastly, last week I ran another test of my own, trying to compare to my old Air 2. Starting with the Air 4 battery already run down to 58%, with backlight at 30%, volume at 30%, in airplane mode, running a downloaded video on Disney+ the whole time, I got 10 hours of playback before the Air 4 died. Figuring it would have lasted around 17 hours had I started at 100%.
This all makes sense as it is consistent with A14 vs. M1 prior battery tests where ever time Apple comes out with a new and all-powerful chip it makes a small, but noticeable, difference to battery life.