Currently we keep 30 days' worth of nightly builds, and also allow builds to be up to 90 days old (the second criteria is used to clean up old feature branches, for example). If we drop that from 90 to 45 days, that will clean up ~90GB of builds between the Firefox and Thunderbird symbol directories.

This does not touch release builds at all, right? What policy do we have there?

Pretty simple.

Release builds are currently saved forever. This only applies to nightly builds. This kicks in when we have less than 30 builds for a particular branch, but they are older than this limit. This happens a lot when we have feature branches that stop getting built, and also when we change versions (because nightlies are grouped by product-version-os).