This is a pet peeve of mine and nothing more, but I've seen it a lot in similar contexts. Logging in "at least three to five times a week" makes no sense. We can log in at least 3 times a week. If we happen to log in 5 times a week, then that would still be at least 3 times a week.
You could argue that "at least" only applies to the three, but then he's telling us to log in a minimum of 3 times and a maximum of 5 times per week. Who knows, maybe's that the case. Given how often the website goes down, maybe they're trying to limit bandwidth usage.
Or he could be telling us that we must log in a minimum of 3 times per week, but he'd like to see us logging in between 3 and 5 times per week. Clearly, this is what is actually meant, but it is not what is written. Now, I know that no one is going to be confused by this. So it is what's conveyed, even if it is not what's written. So why am I complaining?
My reasons are three. First, he's an English teacher. He's going to be correcting our grammar for the next five weeks with varying degrees of exactitude, so it'd be nice if he showed the same amount of care in his own writing.
Second, this is a course designed for writing in the workplace. Like, if you wanted to be a tech writer, you might start with a course such as this one. Well, I've been a technical writer before. And you know what? Imprecise language like the phrase used above is not cool. It leads to mistakes, complaints, and lawsuits.
Third, as a professor of English, he should really care about doing right by the language. Yes, language changes. Yes, the point of a language is communication, and if that's achieved, who cares about anything else.
But here's what gets to me. The English language is a gigantic hodgepodge of words and there are an absolute crap-ton of ways to arrange them meaningfully. So we can communicate precisely if we have sufficient mastery over syntax and vocabulary, but we don't because it's hard. Instead we think whatever we're thinking, vomit out the words we most closely associate with said thoughts, and hope the listener has roughly the same associations for those words, thereby getting the gist of the message across. Blech.