In the past few years, Google has released the first developer preview for the next version of Android around February or March. With the final release coming around August or September (a few months before the Pixel smartphones are announced and available). It appears, at least according to Evleaks, that the first developer preview for Android P is targeting a mid-March release. That would still mean that there could be two developer previews released before Google I/O in early May.
Google typically releases around five or six developer previews for new versions of Android. This helps the Android team get all of the bugs fixed, but also helps developers get their apps ready for the next version of Android, and utilize all of the new features and API's that are coming in this next version. Google also doesn't typically announce the official name of the next version of Android until around late summer, just before the last developer preview (with finalized API's) is available. So it'll be referred to as "Android P" for most of the Spring and Summer. Including during the keynote at Google I/O, where Google will go more in-depth with the new version of Android, which is definitely going to be an exciting part of the event for developers.
When the first developer preview for Android P does launch, don't expect to see a ton of features announced with it. Google will announce a handful of newer features, usually the smaller features, with the first preview or two. But it will keep the bigger announcements for Google I/O when it talks about the developer preview (and also launches a newer preview). But as is the case with any new version of Android, everyone's going to be excited to see what Google has been working on for the past year. In fact, there have already been a number of leaks coming out depicting what Google may be including in this next version of Android, which should be Android 9.0, if recent history is anything to go by. So the next version of Android should be seeing a developer preview release fairly soon, in the next week or two.