Usually, we do announce things here as soon as they are merged. There will often be some delay between that and when you see it on your site, but most will see them within a couple days.
That said, when things get deployed to a given site depends on a number of things, some of which are in our control, and some of which are in your own control.
We are working on making the “manage updates” page in your site a place that helps you more easily answer this question.