Course lengths for antibiotics isn’t well studied. From this article:

In fact, the optimal length of treatment in many common infections is not well studied and may be more than a little arbitrary. One infectious diseases doctor has suggested, somewhat satirically, that most of our current rules for antibiotic administration have more to do with the number of days in the week than they do with robust scientific evidence.

We have a growing and, frankly, terrifying issue of antibiotic resistant bacteria from over prescribing and longer than necessary courses.