To repost and add to my response to your previous question:
You will likely not be able to prevent checking and cracking of this piece as it dries. You have different rates of shrinkage depending on the direction of the grain, and with the cut being kept essentially in it's original configuration (i.e., a section cut from the whole tree), it will want to crack and check like every piece of non-split firewood you've ever seen. Additionally, due to the thickness of the material, you will likely have an exterior layer that dries much sooner than the interior, further exacerbating the checking/cracking issue.
As @keshlam mentions in his comment, the rule of thumb is one year per inch of thickness for drying wood. This is a rough approximation and depends on a lot of factors (species, local climate, how the piece is stored to be dried, thickness, grain orientation, among others). There are quite a few Lost Art Press blog entries on the subject of drying of monolithic workbench pieces you might find of interest (see also this google search for more on the subject from LAP).
In your original post you mentioned cutting out the rot in the center of the log, which I would like to address here as well as it pertains to drying. Removing the rot will not only help keep the rest of the log rotting along with it in time, but it will also help with cracking in that you won't have the center pith. By removing this, you will allow the outer portions of the log to shrink in with less inhibition, thereby (hopefully) alleviating some cracking. Also, assuming you remove the center, I would refrain from jamming another piece of wood in the open space for fear of increasing the log's likelihood of cracking.
However, once the piece has had time to dry and season, you can stabilize the cracks with butterfly keys or epoxy, to name a couple options. Given the thickness of your piece, you may be waiting a while before being able to do this.
Edit: I've included an excerpt below from Roy Underhill's book The Woodwright's Guide: Working Wood with Wedge & Edge to pictorially show how shrinkage occurs in a whole log.