My paper was in "Reviewer invited" state for close to 7 months and recently changed to the status "Under Review". Why did it took 7 months for a reviewer/reviewers to accept to review, or find one who accepted? Is this generally a bad sign? and how much time it may on the "under review" state? Is it recommended to wait further for the review process to complete? Its an inter disciplinary math journal, to which both engineers and mathematicians contribute.
It's a bad sign in the sense that the editor handling your paper is not very active, but not a sign that your paper is going to be rejected.
The difference between "reviewer invited" and "under review" is simply that in the second case, at least one invited reviewer has agreed to review it. So your manuscript is one step closer to a final decision. The bad sign stems from the fact that it really shouldn't take seven months to find a reviewer. It is possible that there are benign reasons for this, e.g. perhaps the editor handling your paper retired and left the editorial board, but the most likely reason is simply procrastination.
It's not possible to tell how long it'll take for the reviewer to finish the review, unfortunately. Whether to wait it out or to withdraw and submit elsewhere is up to you. If you choose to submit elsewhere, know that you'll be starting from zero (and it might take them seven months to find a reviewer too ...). If you choose to do so anyway, tell this journal quickly so they can save everyone's time.
The status “reviewer invited” implies that your manuscript has cleared the initial screening and your paper will be sent for peer review. Finding reviewers usually takes some time as reviewers are busy researchers themselves. Also, potential reviewers may decline the invitation, in which case the editor will have to begin looking for more reviewers. The reasons for declining may simply be too much workload or others, not simply disliking the paper.
Once the requisite number of reviewers has been assigned, the status will change to “under review.” You can wait for some more time to see if the status changes. I'm not too sure, but the number of reviewers probably varied, so you may simply have needed a lot of reviewers.
The time frame for review is around 3 weeks or so based on what i've seen based on some looking around. Then you might have a decision made, which might be a short round of reviews.
It may simply have been a month timelag in updating the listing, and simply difficulty in finding people in quantity (and probably expertise imo) to review the paper. I doubt it is an inherently bad sign, but do write to the editor, after all 7 months is a long wait. Best of luck with getting it published.