Away from the charger the Hololens offers 2-3 hours of active use and up to 2 weeks of standby time.
However just because your built-in battery has run down does not mean the fun has to stop.
The unit charges via Micro-USB, just like your smartphone, and Microsoft specifically notes the device is “Fully functional when charging” meaning that with a simple powerbank, which are freely available in a large number of sizes and capacities (often topping out at 10,000 to 20,000 mAh) the fun never has to end.
With 10,000 mAh power banks now running as cheap as $14, I think Microsoft has guaranteed that, with the addition of a slim cable to your shirt pocket or belt, you will tire well before your headset does.
The Hololens start shipping in the US and Canada on March 30th at a price of $3,000 and prospective developers can apply to purchase the kit here.