It’s anyone’s guess when the next major Windows 10 update will be out
It looked like Windows 10 build 17133 was going to be blessed as the 1803 update, but that plan has been derailed. Though the build was pushed out to Windows Insiders on the release preview ring—an action that, in the past, has indicated that a build is production ready—it turns out that it had a bug causing blue screens of death.
Microsoft could likely have addressed the situation with an incremental update, but for whatever reason, it didn't. Instead, we have a new build, 17134. This build is identical to 17133 except that it fixes the particular crashing issue. Fast ring Insiders have the build now, and it should trickle out to Slow ring and Release Preview ring shortly. If all goes well, the build will then make its way out to regular Windows users on the stable release channel.
When will that happen? That's less clear. The expectation was that 17133 would be pushed out on this month's Patch Tuesday; with the delay, May's Patch Tuesday would be the logical opportunity, though if Microsoft is happy that the build works, there's no particular need to wait.