For those curious about voltage retention in charged cells, I have an interactive chart with measured values over the course of more than a year so far: Long term charge retention plot of laptop pulls
In short, after an initial relaxation period of a few days with larger drops immediately after charging, the voltage drop asymptotically settles to a fraction of a millivolt per day, depending on the cell. In other words, it’s pretty stable.
As for the charge capacity, that isn’t immediately obvious, but since it’s related to the voltage, it is presumably similarly stable.