Are rejections usually quick for papers submitted to journals?
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.