I have been using this Van Der Spek Mega Codex in Undyed Janet Leather since the beginning of March (3 months). It has remained inside the house because I am not carrying much with me these days due to shoulder problems and still working at home most of the time. Despite that, you can see some darkening (more on the front than the back) and marking (which I love). I am considering putting the whole cover in the sun for a week or so to see if the darkening evens out.
I am really loving the feel and texture as it ages and is used.