I think that for running games locally on the Frame, for anything other than games designed specifically to be gentle on a battery
and many games are not, unfortunately
you're also really going to need to leave it plugged into a powerbank. The internal battery just isn't that large relative to what the device can draw.
The battery included on the Steam Frame is a 21 Wh model. The Snapdragon system-on-chip gobbles up around 20 W at full power—that's how much it'll likely use while playing a game locally in standalone mode. From this, we can expect around an hour of playtime without additional charge.