There is nothing conditional about it since the person gives clear instructions: "Tell me when it's ready".
However, the issue is not so important because the future is always unknowable. You can say
"I asked them to let me know when it is ready"
even if you knew that it might never be ready. Depending on the nature of the situation, you may or may not have wanted to openly admit the possibility that it might never be ready. If you wanted to admit that possibility, you could say
"I asked them to let me know when, if ever, it would be ready"