After every volume in my next serial, I want to take a break, since I am planning on continuing it for a few years, and each volume is a somewhat self-contained story with an over-arching plotline. However, I can't decide on a length: one week, or one month? Having a week would minimize people forgetting about it, but with a month, I could easily get the entire next volume done.

Questions to ask would involve the size of your readership, how many readers are you gaining vs. losing? Are you advertising? Marketing otherwise? What's the upkeep on your site (tending to spam, monitoring comments)? If there's a chance you could come back to a clusterfuck, you might want to keep the break shorter. If you draw in new readers with some regularity, then you stand to lose less with a longer break.

In terms of the next volume, do you have prior experience that lets you estimate how much time this takes? How reliably does the writing come to you? What are you going to do if the next volume doesn't come together properly? If a real-life crisis intervenes?

To my recollection, MeiLin Miranda used to have guest authors do shorts in her world during her breaks. I think it helped her maintain her audience.