What’s the opposite of busy?
So, the situation is the following: my girlfriend says she will be busy for some time. So I want to say: "message me when you get free/become unbusy/get unoccupied, please"
What’s the right and natural way to say it? All of these variants sound a bit strange, though I am used to saying "when you get free".

Question

Answer

I am a Native English speaker and "when you're free" is exactly what I would say.
"When things slow down" is another alternative that I would use.

Some opposites (antonyms) of busy are;
"Idle" (Does not work for this one)
"inactive" (Does not work for this one)
"Unoccupied" (Works, but not natural)

Antonyms are words that are complete/total opposites of other words.

Answer

Message me when you can.
Message me when you are able.
Message me when you are free.
Message me when you have time.