It really depends on what the PocketSreen does.
But there are many ways to save energy. As example: you can use as many black pixels as possible because black pixels doesn't use power at all, since it's an OLED screen. You can also adjust the brightness of the screen or turn it off completely if you don't use it. (The Pocketscreen you can see in our Photos & Videos was always at 50% brightness)
The SAMD21 has several Idle/StandBy modes that consume different amounts of energy.
Take a look at page 147 of this datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/SAMD21-Family-DataSheet-DS40001882D.pdf
For reference: the PocketStar survives ~1-2 weeks with about ~1-2 hour daily usage at 70% screen brightness with a 175mAh battery.