Ars Technica's article put it at 1.5hrs in performance mode and over 2 in a power saving mode, but noted that the lower saving mode wasn't enjoyable. Of course it's I am sure that is going to be highly subjective to what you are doing.

https://arstechnica.com/gaming/2026/09/steam-frame-the-ars-t...

Valve confirmed a separate strap with hot-swappable batteries 'coming soon'