I would like to know what does "It's only who knows when I will respond." mean? Is this an idiom?
Background: She is pretty much busy with her work lately so she seldom replies to him. She said, "I was pretty busy lately. It's only who knows when I will respond. Sorry about that." ? By the way, the two are still getting to know each other virtually.
It's quite vague to me but from what I understood, only then she can reply as soon as possible to people who are much closer to her like her parents and the like? It's like she only prioritizes replying to people whom she knows more?
Is my understanding correct?
Any idea is very much appreciated.