The honest answer about when is exactly when Apple is sure that it is ready. The last thing Apple wants to do is to rush an update that has another problem.
Sometimes they tell members of the tech press what the issue was, but not always. For example, there was an issue with watchOS 3.1.1 that caused it to be pulled in December 2016 and it was more than a month before there was an update to 3.1.3 (there was never a 3.1.2), and I don't think that Apple ever reported why 3.1.1 was having issues. So, it could be a while before there is an update, and there may be no official explanation.