According to David Lapham, the book will be ending with issue 18:
Got word yesterday. 18 will be the last. There will be plenty of mind blowing shit in every issue till the last. I'm not going to try and force my "ultimate" ending in there but we will have an end, and as this is a series of endings, I can hold out hope that one day I may see it through to the end I had intended be the end, end..
A real shame, because, as I mentioned the other day in my random thoughts post, this is (was?) my favourite (formerly) ongoing series right now.