I have the feeling that's not (only) about how many examples of plagiarism are presented, it may be also related to the number of reports (and maybe reporters).
One of my (few, sorry) plagiarism reports stayed there for quite a while (more than 2 weeks) until that account was banned, although he has made plenty of plagiarized posts.
On the other hand, checking properly such an user requires more time than removing a spam/shitpost and the overall damage is big, hence the mods do have to pay proper attention. So I can understand that they may not always be "in the mood" for such a job.
I would surely not count on that!