I have to mention that about 6 weeks after the submission, its "status
date" has been changed but its status not!
The status date changed because, either the editor sent the paper for review or any of the reviewer accepted to review the manuscript. Either case is possible. The status changes/date changes occur when some step has been taken by the editorial side or reviewer side.
May you please tell me how long I have to wait for the decision about
I guess, nobody can give you a numeric estimate of this duration. It is completely dependent on the reviewer and their response to the editor's reminders. As a reviewer, we get around 4 weeks to review an article. However, they may remind us once or twice or sometimes give us an extension to do so. If the reviewer is very popular (bigshot), the editors sometimes don't care much about the timing rather focus on getting a strong and bias-free review from such reviewers. That takes time.
I've checked the paper status in journalinsight.elsevier.com. For the
journal, the first decision speed time is about 7.2 weeks. But my
paper is still under review after about 3 months.
I have been waiting for last 6 months (and counting) to get the very first review on my paper; not in this journal, but something similar. If you are so curious to know, you should send a very gentle email to the editor asking about it or just a hint to remind him about your article. Because, in a pile of many articles under review, the editors sometimes may not look into your article. Probably something like this:
Could I ask you about the present status of the review of my following: submitted manuscript?
Title: Knowledge-Based Analysis of Review period of the Journal X (ID: KBS-16-xxxxx)?
Remember, editors are busy persons; he many not respond to your email too. And, Computer Science/Mathematics journals often take a lot of time for review purpose -- This is well-observed fact. No proof of this hypothesis is required!