When are newly released albums available on Apple Music subscription service ?

It SHOULD be immediately. Why? Which album/artist are you referring to? Sometimes, an album/ep/single drop may be late as seen with Troye Sivan's single, Dance To This. It dropped 5 hours late back in June. Hope this helps! Also, the artist SHOULD have announced when an album or EP is coming out of the Pre-Release phase.

It is up to the artists and their representation to release content to the iTunes Store and/or Apple Music. If the artists want to release it in the Store before the streaming subscriptions, then that is their choice.