* For length: The opposite of "long" is court.
* For time: The opposite of "long" can be court, brève, or rapide.
* For duration: The opposite of "long" can be court, brève, or rapide.
Here are some examples:
* Un pantalon long (long pants) - Un pantalon court (short pants)
* Un film long (a long movie) - Un film court (a short movie)
* Une longue attente (a long wait) - Une courte attente (a short wait)
* Un long voyage (a long journey) - Un voyage court (a short journey)
* Une longue histoire (a long story) - Une histoire courte (a short story)
The specific word you choose will depend on the specific meaning of "long" in the sentence.