Thank you! But if I include the word “short”, should I say “A teacher shouldn’t remain on a topic for too long or too short a time” or “A teacher shouldn’t remain on a topic for too long or too short”? or “A teacher shouldn’t remain on a topic for too long or too short”? Do I need the word “time”? If so, do I need the article “a” before “time”? What about “too long/short OF a time”? Thank you very much!!

I agree. The first one is correct, but I would say “She has been talking for too long.” The “time” part is understood. I would say, “A teacher shouldn’t remain on a topic for too long or too short a time” but other people may think differently.

I would use "She has been talking for too long" without the word "time". And if it is not a conversation, but a lecture, a speech or something like this, I think the correct sentence is: "She has been speaking for too long."