IEEE TVCG length of review process
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.