Currently we don't release regularly. Whenever we think it makes sense to release a new version we do it. You might want to ask in our Slack channel external-dns when the next release will come out.
When releasing a new version of external-dns, we tag the branch by using vX.Y.Z as tag name. This PR includes the updated CHANGELOG.md with the latest commits since last tag. As soon as we merge this PR into master, Kubernetes based CI/CD system Prow will trigger a job to push the image. We're using the Google Container Registry for our Docker images.
The job itself looks at external-dns cloudbuild.yaml and executes the given steps. Inside it runs make release.staging which is basically only a docker build and docker push. The docker image is pushed gcr.io/k8s-staging-external-dns/external-dns, which is only a staging image and shouldn't be used. Promoting the official image we need to create another PR in k8s.io, e.g. kubernetes/k8s.io#540 by taking the current staging image using sha256.