Number of days of reading time has always been a manipulated number (by everyone).
1st there is the definition of a 'day'; 3Hrs is one number I have seen used
But there is more
How many page turns per hour affects CPU usage more than idle time (while reading the page).
Back light usage?
Audio usage (will prevent full CPU sleep), Blutooth...
1200mA for an eink device used for visual reading should be plenty for days (Ya gotta sleep sometimes, so put it on charge then and quit worrying

)