When it's ready.
Atlassian don't give specific dates until they are happy that something is ready to ship (usually only a few days before). You are going to have to guesstimate it from the activity you can see in public - the version of OnDemand Confluence is a useful indicator, Atlassian blogs, the age and complexity of the early access versions available for download and so-on.
You can also look at https/jira.atlassian.com/projects/CONF/versions/50804 to get a clue.
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