One objective would be to convince those colleagues who tend to do a lot of elaborate string manipulation using only built-in string functions—a common anti-pattern I've observed—to use regular expression pattern matching instead.
This makes me wonder, is your intention to really teach about regular expressions, or are you pushing a political agenda?
How would you teach a class on regular expressions? What would your approach be?
That depends. 4 to 8 hours isn't a lot. I've done regexes as part of Perl training course, and although I typically spend 4-6 hours of a 4 to 5 day training on regular expressions, I only barely scratch the surface.
How much do your co-workers know? Do they know the concept? Are they familiar with at least v8 style regexes? (as used by grep, sed, vi, awk, etc). They you can skip the basics and focus on more advanced things. Otherwise, 4-8 hours is just enough to do the basics.