I actually gave it the old "start from full charged and see how long it goes" test last night.
Started it at approx 10.30 pm last night. All flac files on random into an HD600 at low to moderate volume. It quit at just before 10am this morning - so approx 11.5 hours. That's pretty good. Nowhere near the Studio - but OK for a long haul flight.
BTW - charge from a wall-wart (from empty to full) was pretty much 4 hours.
Idea for Fiio for an add on sale. A perfectly size matched add on battery recharging unit (rechargeable). So that way if you are running low - just throw the battery pack in place and recharge the X5. Food for thought anyway.
Charge from a wall-wart (from empty to full) was pretty much 4 hours? What is the output rating of the charger? 1A or 2A? I believe Fiio recommend a 2A charger, which imply X5 support high-current quick charging, and 4 hours seems to be a bit longer then expected.