I just started reading Cress (Lunar Chronicles 3), which is great so far, but the only problem is I can't remember most of what has happened! It must be well over a year, perhaps maybe two years, since I read Scarlet, and a few months before that I read Cinder. Scarlet must not have made a huge impression on me, because I completely forgot about Thorne and Wolf, and where exactly Cinder was at this time.
Thank God for recaps at the beginning of books. Usually I hate this, but this time I am really glad there have been enough references to past events that I can (sort of) remember what has already happened, and piece together the parts that just elude me.
So my question is, how long is too long to leave a series for? I guess it depends on your memory, and on how much you previously enjoyed that series. A year seems to be the accepted time frame by publishers, but I believe that most people's memories aren't quite that long. They remember the jist of the story, but not all of it. Details woven into the plot can be easily missed by those, like me, who can't remember a name or seemingly insignificant events.
Needless to say, I'll be trying not to wait so long between the next books.