It doesn't necessarily mean you are a lousy writer or that your book idea stinks. Also, good writing takes a tremendous amount of practice. That was certainly my case. Many projects need continual refinement before they are ready for publication. So take any critical comments to heart if an editor takes the time to give you a detailed commentary.
Response time varies.
If I know a project is not for me, I try to return it quickly with little comment so the writer can get on to the next likely publisher. Three months is a fair time for response. However, it could be much longer. A gentle note of enquiry in the mail beyond the five month period would not hurt. However, badgering phone calls will probably do more harm than good.