After reading the review from @TheoS53 and being somewhat surprised at the findings of 12 hours+ playback time, I decided to do my own timings.
My perception had been around 7 hours. However, I mostly used it to drive the Walnut as an amp, often on high gain, and usually charging when it had been on 1 bar for a bit.
So here goes. Low gain, through my Monk+ Espresso, which are 64 ohms, not massively difficult to drive but not exactly low impedence either. Volume was the loud end of listening volume, ranging from 55 to 63 out of 80 depending on the recording.
Outcome - 12 hours 19 minutes!
Really surprised, I've been doing it quite a disservice battery-life wise.
Interestingly, I was down to 3 bars from 4 in less than 2 hours, and down to the last remaining bar within 8 hours. But it just kept running and running!
So..the charge indicator isn't massively accurate, and more importantly, running time is pretty good after all!