Recently it happened to me twice that I had a paper which was under review for more than one year (first round) and that all inquiries about the status of the paper went unanswered by the handling editors. Both cases were reputable mathematics journals but not among the very top journals (not the Annals or similar).
Is it reasonable to withdraw the submitted paper in this situation and to submit it to another journal? What is a reasonable waiting time in such a situation, i.e. very long time under review (more than a year) together with an editor who does not respond at all to inquiries on the status?