We upgrade according to the following cycle:
Internal staging > Client staging > Client production (and sandbox)
We maintain the following conditions for advancing a new release across the environment types:
If items are fit for release to clients, they will be deployed to your staging. This will be communicated in our release notes.
Releasing to your production or sandbox requires a formal request for change, and acceptance testing has to be passed by you. A request to upgrade is considered equal to completing acceptance testing.