See the Roadmap page for the official schedule.

You'll find there the currently opened Milestones, but you can use the filter panel at the upper right corner of the page to select "closed milestones" to get an overview of the whole project's life.

Note that the planned release dates are not bounding, they should only be taken as an estimation. We're also using the dates as a way to order the milestones.

See also SeaChange for a general discussion about what users (and developers) would like to see happening in Trac.