Leaving this here as a reference point for anyone else who is curious.
I just got a T-Deck to have as a reference platform while working on the PicoCalc; printed a case for it and I put in a 2000 mAh battery (the PicoCalc has 6400 mAh of battery).
I was curious, so I ran some tests to see how long the battery life lasted while “being actively used” — keyboard backlit, screen brightness usable, and running cycles of benchmarks.
The T-Deck lasted about 5h 35m, and the PicoCalc just over 16h.