Thank you for posting this. I hadn't looked the online manual!
So my 28 hours result for FLAC CD quality is about right considering that 96kHz/24bit is 25 hours. So it looks like I have a healthy ZX1, despite leaving it connected to my PC all weekend which seemed to result in it never stopping to try and charge.
Interesting that they don't dare to quote how WiFi affects those times. I can see from my own tests that it really hurts it, more so than Bluetooth.
Wifi hurts battery life whatever the device may be, from laptops to smart phones. So, nothing unusual if it does have an adverse impact on the zx-1.