Google has released the first developer preview for Android O today, and they have also laid out the roadmap for this new version of Android. Users can expect to see the final release available in the third quarter of the year (between July 1st and September 30th), which is pretty similar to the time table that Google used last year with Android Nougat. Google will also release four developer previews, so that's three in addition to the one released today. That's about one per month, however, the graphic that Google uses shows that the second developer preview won't be available until May, with the third coming in June and the fourth in July, before the final release. For reference, Android Nougat was officially released in August of last year, so we could be looking at the same date again this year.
Currently, Google is classifying the first preview as the initial release and an alpha. While the second preview graduates to beta while being an incremental update. The final API's and official SDK will be available in the third preview, as well as Google Play Store publishing beginning. The 4th preview will be pretty much the final release, with just a few changes before the release goes to AOSP and OTA's begin rolling out. So those that are wanting to check out the latest version of Android, you'll want to stay away until around Preview 3, unless you are not using this on your daily driver.
Google is making the preview available for the Nexus 5X, Nexus 6P, Nexus Player, Pixel, Pixel C and Pixel XL devices. Now that doesn't mean that these are the only devices getting updated to Android O later this year, but it does mean that these are definitely getting updated. We've seen with previous previews that not all the devices in the previews are the only ones getting the update. So keep that in mind. For instance, the Nexus 6 was not part of the Android 7.1 preview, but did get the update, so there is a chance, although slim as the Nexus 6 will be closing in on 3 years old by the time the final release of Android O is available.