Are rejections usually quick for papers submitted to journals? When submitting papers to journals, I understand from experience that acceptance can take quite some time (on the order of several months), even when a reviewer is immediately assigned to the paper.  But can a referee usually tell fairly quickly if the article should be rejected?  Are there any horror stories of waiting several months after a reviewer agrees to look at your paper, only to get a rejection letter?
The answer depends on the journal, the paper, and the reviewer(s). You might hear relatively quickly if there is a trusted reviewer available quickly and he/she isn't too busy with other work and the paper is either especially good, or especially bad. Often the bad is easier to judge than the good, of course. The longest reviews are the intermediate papers, especially if different reviewers have different opinions about it. "We want to publish this. Do we have room in an upcoming issue? We have a lot of competing papers. I love it. I don't think it's quite ripe yet. Etc. Etc. Etc."

Note also that the reviews of intermediate papers (and even some judged to be quite good) may come with some advice for improvement that will, perhaps, lead to acceptance after additional review. This will depend on the journal, of course.  

However, don't wait to hear the judgement of the reviewers. Continue work on the project, working to improve both the result and its presentation. Work to extend the result.