I guess I never really looked for a new battery, tbh... My experience is that typically, replacing a non-standard (AA, AAA, etc) battery ends up being not worth it in the long run.
As for how long the battery lasts... Only a few days (5, sometimes 7). It seemed worse when reading .txt files, instead of .epubs.
There's also just a little bit (maybe more than a little bit) of "I want a new toy".