How much time does each stage of paper in Elsevier Editorial System (EES) take?
Each electronic manuscript handling system typically has a set time frame for each part of the review-revisions process. The systems have default values which, I would assume, are what most journals use. It is, however, possible to custom make the different time periods for each part of the process.

1.Submitted to Journal:
This usually does not involve a lot of time and a reasonable time frame might be 5-10 days depending on factors such as workload, time of year etc. The step might involve inspection of the quality of the submitted material, but not necessarily.

2.Editor Invited (Conditional - this step may not occur):
This may again be another 5-10 days and may be affected by workload etc.

3.With Editor:
The main problem that occurs here is difficulty to assign reviewers. The editor may also be overloaded with work. The time frame is likely something like 3 weeks but will vary.

4.Under Review:
Each journal has a time frame and  3 weeks is what I am accustomed to in my field. Reviewers that accept reviews and never return them is a problem here. How quickly the editor tries to assign other reviewers to speed up the process will vary from person to person and journal to journal.

5.Required Reviews Complete:
I am not sure if you actually mean revisions here because typically the author gets about 3-6 weeks to revise the manuscript. This is another point where delays can occur but they are self-inflicted.

6.Decision in Process:
The decision process may require some time since the editor needs to read the revised manuscript and the revisions/rebuttal from the author to make a decision. In many cases there is cause for another round of reviews which would loop back to 4 and 5.

7.Completed – Accept:
7 could be 6 if the manuscript gets accepted after one round (rare in many circumstances).

Once accepted, the manuscript goes to copy editing and type-setting. This may be a quick process depending on the resources provided by the publisher. Journals using LaTeX have the advantage of having print ready proofs basically at the turn of a switch.

To sum up. Each of these steps can take various amounts of time depending on the working conditions at the journal as well as the expedience of reviewers and authors. I think the time frames I have outlined are reasonable but that does not mean all journals will be that quick (or slow, depending on your view).