Since the engine stabilized and got much more future complete already we work with a varying release cycle, normally around 2 months. Please check the calendar below for the next dates. See blog post about activity in Activiti project for some more insided.
Then there is 1 week we call the release-week. The release week starts on the code freeze day. On the morning after the code freeze day, a tag is created for the release. It is assumed that all coding work has been completed by that time, but practice learns that this should be checked with the group ;-) After the versions are updated on the branch, we start with pointing the continuous integration jobs to the branch and start smoke testing. Once that is OK, the publishing of the release can begin.
This calendar shows the release dates and also JIRA Freeze and Code Freeze dates for the releases.
Here's the Activiti project in Jira. That is the official reference. Below is that (not yet) list of issues ordered by priority for planning purposes. The list below in this page might be out of date. You can also find the [roadmap of releases in Jira|http://jira.codehaus.org/browse/ACT#selectedTab=com.atlassian.jira.plugin.system.project%3Aroadmap-panel].