IEEE TVCG length of review process We recently submitted an article to IEEE's Transactions on Visualization and Computer Graphics journal. It is the first time I submit to a journal so I am a bit unfamiliar with the process, being used to conferences. 

In their website they mention that the review process usually takes between 6 and 12 months. I am wondering, does this refer to the time taken by all reviewers to take a single iteration on the manuscript or, does this include more review iterations? I.e. does this time take into account also the time needed by the authors to revise their manuscript according to their comments and resubmit it? How much time does one have to do these revisions? Provided that it is not rejected...
Review time is usually time from submission to final acceptance (ready to print).  After you submit most journals will assign your submission to a group of reviewers based on their specialties.  Those reviewers will read your paper and write up any comments, questions, or concerns.  They also typically issue a recommendation as to whether the paper should be 'accepted', 'provisionally accepted', 'changes requested', or 'rejected' (my terms).  All of that is uploaded to a central website, and when everyone has responded those comments are sent back to the authors.  If it wasn't accepted outright, you'll usually have a chance to address comments or concerns by making modifications (clarifying graphs, numbers, etc.) and resubmitting for reevaluation.  This can go on for a number of rounds of back and forth.  If your paper is eventually accepted, you are notified and sent a proof of what it will look like printed.

Now sit back and watch the citations roll in.

IEEE's TVCG process may differ a bit.  I'm familiar with publishing CS papers, but most of my publications are in Bio.