The college may have a different schedule, say 45 minutes between each class rather than the 60 minutes I have supposed above. I’d be inclined to stick to 60 minutes and let the school adjust its schedule.
If your talk is 40 minutes, you won’t be presenting OR GETTING the full value by eliminating some of your material or by eliminating the feedback of questions-and-answers.
It is the Q&A dialogue that informs YOU of the client’s pain, problems, needs and wants. If you don’t get the Q&A you don’t get the client. You are a disposable source of knowledge.
Why would a college rep want you to address the students? To impart knowledge to the students. Doesn’t the college have someone to do that?
You are getting the college off the hook by becoming a walk-on-walk-off instructor, with no overhead for the college. What a bargain!