Google hasn't officially announced a release date for Android 7.0, but a look back at the past suggests that we will probably see between August and November. I know that's a pretty wide timeframe, but it's probably going to be sooner rather than later.
Looking at past Nexus releases, the flagship devices that Google uses to showcase each new version of Android, we see that the Nexus 6 was released in early November 2014. The Nexus 5X and 6P were released on September 29, 2015.
There are signs that the new Nexus, and the new version of Android, will release slightly earlier this year. There have been a ton of leaked photos and info about the 2016 Nexus devices already, and the final pre-release version of Android is available to devs now. This suggests that we will see a slightly earlier release this year.
Personally, I expect a late August or early September release.