So... I've been really bad lately... I haven't read from my Clara in 3 weeks!
I wish I could provide some more data points, so I'll say that exactly 3 weeks ago I charged my Kobo to 100% on a Thursday night, and took it with me for a weekend trip planning to read. I have the sleep setting set at 10 minutes and never shut down enabled.
I checked the battery % exactly once per week by taking the device out of sleep mode by opening the clara's cover, which causes a page refresh of the book cover art screensaver to the home screen, clicking on the battery % and noting what it was, and then turning it back to sleep and causing a page refresh to the cover art screensaver. I recall at the end of the first week it was at 92%. At the end of the second week it at 86%. And now at the end of the third week it is at 77%.
My device is really only 6 weeks old too, so the battery is very fresh.
But it seems that while in sleep mode, you lose roughly 1% battery per day. At this rate the battery could last for at optimal 100 days, or roughly 3 months.
Side note : Auto power off should retain the battery level much better. Perhaps weeks could go by and the battery % would barely budge.
Kobo stated battery life is
"weeks*" where they stipulate "dependent on individual usage", but which is also what Amazon states for their paperwhite.
Nook states that their glowlight 3 can last for 50 days on a single charged, and qualifies their claim by stating 'based on half an hour of reading per day'.
Quote:
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Based on 30 minutes of testing per day and 1 page refresh per minute with Glowlight at 10% brightness and wireless off. Battery life tests conducted using specific units tested for 25 days with an average drop in power of 2% per day. Actual results may vary based on device settings, usage, and many other factors.
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I'd tend to think that all 6" eReaders with an eInk Carta HD screen, similar if not identical cpu and memory, all of roughly the same dimensions, would be matched to roughly the same battery requirements. Therefore, I'd extrapolate these quotes to apply to any of the devices interchangeably.
If Kobo says weeks, we could be generous and apply these rules to any of the readers.
HYPOTHESIZED:
I estimate that every 30 page refreshes drains something less than 1% battery.
I estimate that 30 minutes of glowlight at 10% brightness drains less than 1% battery, but combined with 30 page refreshes equals about 1% battery drain.
Factoring in the battery life lost per day sleeping, you could get a perfect ideal 50 consecutive reading days before the device requires charging on a new battery.
I would also recommend that if you know you will not be reading for consecutive days to turn the device off and power shutdown, to save battery. Otherwise you will not get 50 reading sessions out of a single charge, or in other words, almost a month and a half of reading. Of course, if you're spending more than 30 minutes reading per day, you'll get less. At one hour per day you'd get 25 days, or just over 3 weeks. If you read 2 hours per day, you might only get 12 days, just shy of 2 weeks. This is all just extrapolation though.
It's also fun to think that if you read at one page per minute, you would be able to read
(1 page/minute)*(30 minutes/day)*(50 days/charge) = 1,500 pages.
And if the average book is 500 pages long, then you could read roughly 3 books per charge.
TL;DR
So, in response to this OP,
MEASURED::
my Clara loses 1% battery per day while in sleep mode (tested over 21 days).
GUESSED::
You lose barely any battery when the device is powered off. I'd guess that Clara is similar to the Nook Glowlight 3 and would last 50 days when reading 30 minutes a day with dim front lighting.