"Ok, in half an hour"
Translation:D'accord, dans une demi-heure
Bon, dans une demi-heure looks and sounds fine to me. Quite a usual way of expression.
The number of occurrences of "bon" in your teacher's speech would determine if it was a tic, I think.
Some say "vous savez/tu sais" (you know), others repeat "euh" (er), I believe we all have our own tics.
It would be a pain, but duo really ought to universally require dashes (or universally ignore them completely).
By requiring the dash only sometimes it is too easy to get in the habbit of omitting them.
Duo seems to forgive the omission in questions, but not when two words become one. In other words demi-heure, quatre-vingt, etc need the dash to be considered correct. Est-ce, tais-toi, etc Duo will forgive you for leaving out the dash, but you should learn to type them anyway, as they are actually required by the language.