According to the latest information bits, Adeptus Mechanicus Skitarii is a 3 week release. With this week's contents already known, it would seem appropriate that we might see two more model releases next week to wrap it up. Especially the missing Onager Dunecrawler, and possibly an HQ unit.
Consider this pretty solid, but as always until we see something official, it must be considered a rumor.
via Mauler in the Comment Section of Faeit 212
My local GW manager said that the Skitarii release will sit roughly between Tempestus & full codex in size. They've got a three week release schedule so that fits in if they get another 4-6 boxes and a book.
He also added that there should be a proper Adeptus Mechanicus codex at some point but he didn't know when. "Three weeks and one book for now..." were his exact words, pretty much.