The developer preview has been planned to be released in three phases. The first in the series has already been out. It is available now for Developers and the owners of Nexus 5, Nexus 6, Nexus 9 and Nexus Players.
The preview will be available for the devices that have been flashed with a preview build before. Google has stated that it will also make the preview available for manual download.
The first preview has already been released while the second one will come in late June or early July and the last preview will be up in late July. The update is expected to be released in the third quarter this year.