The V8 release process is tightly connected to Chrome's. The V8 team is using all four Chrome release channels to push new versions to the users.
If you want to look up what V8 version is in a Chrome release you can check OmahaProxy. For each Chrome release a separate branch is created in the V8 repository to make the trace-back easier e.g. for Chrome 45.0.2413.0.
Branches for a Canary normally look like this
Every week a new Dev build is pushed to the users via Chrome’s Dev channel. Normally the deliverable includes the latest stable enough V8 version on the Canary channel.
Branches for a Dev normally look like this
Roughly every 6 weeks a new major branch is created e.g. for Chrome 44. This is happening in sync with the creation of Chrome's Beta channel. The Chrome Beta is pinned to the head of V8's branch. After approx. 6 weeks the branch is promoted to Stable.
Changes are only cherry-picked onto the branch in order to stabilize the version.
Branches for a Beta normally look like this
They are based on a Canary branch.
Roughly every 6 weeks a new major Stable release is done. No special branch is created as the latest Beta branch is simply promoted to Stable. This version is pushed to the users via Chrome's Stable channel.
Branches for a Stable normally look like this
They are promoted (reused) Beta branches.
The tip of the same branch that Chrome's Stable channel uses.
We often backmerge important bug fixes to a stable branch, so if you care about stability and security and correctness, you should include those updates too -- that's why we recommend “the tip of the branch”, as opposed to an exact version.
As soon as a new branch is promoted to Stable, we stop maintaining the previous stable branch. This happens every six weeks, so you should be prepared to update at least this often.
Example: The current stable Chrome release is 44.0.2403.125, with V8 220.127.116.11. So you should embed branch-heads/4.4. And you should update to branch-heads/4.5 when Chrome 45 is released on the Stable channel.
Related: Which V8 version should I use?