When to retract a submission due to excessive length of peer-review process 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?
There can be multiple reasons for lengthy review process.
1) Your work is very novel and its hard to find a reviewer.
2) You submitted your article to a journal who's scope is not fully relevant to your research area. According to my experience this is one of the major reason. 
3) The ignorance of handling editor. 
 First make your own assessment if your work is very novel or if you have submitted to a journal who's scope is not fully relevant to your research area in both cases you may expect a much longer review process. 
if its not the case 1) and 2), then Check the average time of first decision for recently published article, and contact handling editor after passage of average expected time. If Editor don't reply for another extra 25%~30%  of average time, then write a withdraw letter and mention the unexpected delay as a reason for withdrawal to EiC.