I pretty much always get what is advertised, if I were to go through all that pesky math. But I have had kindles since 2008 and I read a lot. Besides the K1 model, all my kindles, when I use them to read, need to be charged at 2-5 days. Never have I had a kindle last longer than 5 at most. If I use that kindle to read that is. I don't obsess about the time when they sit in a drawer. I plug them all in once a month, or when I remember. Can't really read on more than one at a time.
I had a few weekend binges when I was ill and all I could do was read to get my mind off pain and 2 days was max then.
. That was like reading all day long basically.
I use light at 8 always, don't use auto light. I have wifi mostly off, but sometimes its on and I just don't care.
I see a plug sticking out of a wall somewhere, I'll stick it on while mopping the floors, or cleaning the bathroom, or making breakfast, go shopping, etc. Its that fast.
Now the K1 was a special unicorn. I had to read while plugged in with that one all the time. I think the SD card stuff used more battery and it just wasn't as efficient. Searches took 8 minutes. Yep, I timed that back in the day. Cause I had like 2000 books on the SD card.
It still was and is my favorite kindle as it got me reading again.
I guess the short form of all this babbling in my post is I just read. Plug in when I notice it needs it and read some more.