One of the unfortunate side-effects of publishing eBooks is release dates aren't known, usually, until you're done with all the edits, line edits, galleys, etc. So that release date is dependent on how efficiently the the team can work together. (I say team, because it really does take a team to put together a book for release, but that's another topic for another day). It also depends on how much the original manuscript needs treatment. So there's no one factor that goes into getting a book out onto the street for release.
Here's a recent example from my own personal experience. I signed a contract with Decadent Publishing on July 31, 2011 to publish Slow Break, the 4th Sypricon Masters series book. I received the cover art September 4, 2011 and the first round of edits September 5, 2011. Those edits were returned about a week later and I received line edits September 24, 2011. The release date was then set for December 16, 2011.
Seems pretty straight forward, right? Here's a comparison. I signed a contract with Ellora's Cave on October 26, 2011 to publish Hard as Stone, a standalone m/m paranormal erotic romance. I received the first round edits on October 31, 2011. I received cover art on November 7, 2011, just about the time I returned the first round of edits. Received the second round to tidy up a few places on November 15, 2011 and returned those on November 21, 2011. Release date was then set for December 14, 2011.
So, two books, two publishers, with two very different timelines but the release dates are back-to-back. This is a good example of adjustments that have to be made and how no two publishers are the same. Actually, I don't think, out of any of my books, I've had identical processes from contract sign to release date. Each has had their own unique timeline. But all this means because of the long lead time for Slow Break, I already have promo in place for dates around the 16th. Now I have another book, with a shorter lead time, to promo at the same time. Luckily, both books are m/m stories and can be promoted together despite the sub-genre differences in them.