I have, as you might suppose, been getting a certain amount of agitated mail following publication of ECHO, [g] all centering on one question: HOW long is it until the next book will be out?!? (This accompanied by various pleas of increasing age or senility on the part of some readers. It's not that I'm not sympathetic, and I _am_ flattered--but the fact that you think you might not live long enough to read the next book really can't make me write any faster. Look, just exercise and take your vitamins; you'll be fine.) As to the question...
It wouldn't do much good to speculate; I'm always wrong. The books are invariably longer and more complex than I think they'll be, and _always_ take longer to finish than I think they will. Sufficiently so, that anyone who's had the misfortune to work production on one of the OUTLANDER books emerges from the experience with several new white hairs and trembling visibly in every limb.
General observation is that in terms of actual research/writing/etc., it takes me _about_ two to two-and-a-half years to write one of the big novels of the main series. That's baseline. However...
A) The books get increasingly popular. While this is, generally speaking, A Good Thing, it has side-effects. There's a HUGE demand on my time, in terms both of physical travel and in-person appearances, and the smaller drag created by endless email, propositions, invitations, cover quote pleas, and now--with the upsurge of online venues available--the demand to create TONS more subsidiary material for blogs, online chats, interviews, phone apps (don't even ask about that one; the publisher just suggested it and I have _no_ idea), enhanced e-books, viral animated videos, etc. I lose at least two/three months a year, if not more, to this kind of stuff (much more, in years when a new book is out and several different countries want me to come and tour. So far this year, I've done Scotland, the US, Canada, New Zealand and Australia--for a total of two and a half months _just on the road_--and will be doing the UK in January, Germany in March, and some miscellaneous Scottish stuff [g] in August. Three European trips in one year? It has its fun aspects, but man, it eats up your work time.
B) I don't work on one project exclusively, for the most part. I never have. Being able to switch mental tracks keeps me from ever having writer's block, and it's good for the separate projects, insofar as pushing one off the front burner while I deal with something else lets the first one simmer peacefully on the subconscious's lower heat--so the result has _lots_ more flavor and subtlety than it would if I cooked it at a furious boil and splashed it scalding into people's bowls (cf. James Patterson, if you want to know what _that_ looks like).
So (for instance), while the pub date for ECHO was four years after that for ABOSA, it doesn't mean that I spent four years tapping away on ECHO without cease or respite. I also wrote (and published) two other books in that time (LORD JOHN AND THE BROTHERHOOD OF THE BLADE, which was a complete novel, and LORD JOHN AND THE HAND OF DEVILS, a collection of novellas), wrote the complete script for THE EXILE (the first "Outlandish" graphic novel), did work on a contemporary crime novel (about half-done at present), worked on THE OUTLANDISH COMPANION, Volume II (rudimentary stuff, but still work), consulted on a film script, and poked around in the research material, coming up with bits and pieces of other, future books (like the first Master Raymond) that I haven't started working on in any focused way, but that are waiting on the sideboard for their turn.
So I have no idea when Book Eight'll be up. Not for at least two and a half years (the publisher does need _some_ lead time to actually print the book), but that's _all_ I could tell you for sure.
Now, I _can_ say what I plan to be working on during 2010 (putting aside all the roaming around the world stuff, promotion for the graphic novel, and Eldest Daughter's upcoming wedding): I'll be feeling my way into Book Eight, working in a serious way on LORD JOHN AND THE SCOTTISH PRISONER, ditto the OC II (because I'd like to have that one done before the end of the year), and seeing if I can pull together the contemporary crime novel (which is about half-done, and I think I know where the other half _is_, more or less).
You will get a couple of things in 2010, though: 1) THE EXILE. This is the much-talked-about graphic novel, due out September 28!, 2) a new Lord John novella, "The Custom of the Army" (due in March from Tor books, in an anthology titled WARRIORS), and (probably) 3) "A Leaf on the Wind," a short story in a Penguin anthology titled STAR-CROSSED LOVERS. This one tells the story of what really happened to Roger's father, for those with a particular interest either in MacKenzie family history and/or time-travel. [g]
So you will get something to read, and fairly soon. As for Jem...don't worry about him; he's a pretty resourceful little guy.
Speaking of that, though--which is the storyline _you_ most want to see addressed Right Up Front in Book Eight?