To expand a little more.
The estimated power consumption is ~19.5W (assuming that is in W/hr) or ~1000 mAh. Assuming your battery can handle a discharge rate of 1mAh then 2-3 hours is ok (maybe). On that note the battery will might have a voltage that will drop over time making it inoperable at some point prior to the battery is fully discharged and may cause the NUC to behave erratically or break.
Would I do it: Why not. Maybe you'll want to ensure your pack has some filter on it to cut out if it's voltage drops too low or it's discharging too quickly (overheating) - most should have something.