Two weeks ago, Microsoft announced the Windows 10 May 2019 Update, along with some major changes to the way feature updates are delivered. While the May 2019 Update is coming to the general public in May 2019, those with MSDN subscriptions found today that ISOs for the feature update are now available.
It's a new world for Windows 10 updates, so there's no real norm to compare this to. On one hand, this is around when previous spring updates would have shown up; however, those shipped in April. Microsoft promised to give Windows 10 version 1903 a whole month to bake in the Release Preview ring this time, which is why it's coming in May.
Earlier today, Microsoft announced the availability of the Windows 10 May 2019 Update SDK. With that in mind, it probably makes sense to offer ISOs to developers, or at least those with a paid MSDN subscription.
Of course, if you don't want to shell out the cash for a paid subscription, you can always use the RTM build that's available via the Slow or Release Preview rings of the Windows Insider Program.