The HHVM (HipHop VM from Facebook) has released an update on their blog today discussing some of the long term support they plan to provide for the project and what kinds of things it will involve.
HHVM is known for its very intense and quick development pace: currently we ship to GitHub the exact same code we use to run the Facebook site within minutes of every commit, and we release a full version every 8 weeks. That is great and at the same time scary if you are trying to build a business or infrastructure around it. The HHVM team at Facebook understands that in order to reach every corner of the PHP landscape our users need to have some sort of commitment, in order to plan their deployments accordingly and to know how upstream will react to security and stability fixes in already released versions, for example.
Starting with HHVM v3.3, they'll be supporting two major versions at all times. They provide a table of versions and dates to give you an idea of when the support coverage period is and when they'll end. There's also some discussions about the packaged released for the various linux distributions and what kinds of updates might be included in the long-term support (LTS) updates.