Really hard to say. Originally I was thinking it would be this summer like the previous Nexus 7 tablets. But previously Google released a new version of Android in the summer to go along with it.
Android L seems like it won't arrive until this fall. And it seems like Google could very well wait to release the Nexus 8 or 9 so they have new hardware to launch with the new OS.
Though it's possible it could still launch this Summer and maybe Google instead launches a new Nexus phone a bit earlier than normal with Android L in the Fall. Like I said, hard to say.