The purge is P/Rs is configured to happen after a number of days since the last analysis of it. We also show how many days have been since the last analysis in the "Branches & Pull Requests" configuration page for each project.
However, the criteria that is actually used to purge branches and P/Rs depends on the "updated_at" column of the PROJECT_BRANCHES table. The idea was that this value would only updated when the branch is analyzed, but in practice that's not the case. For example, it is sometimes updated when there is a change to that table in DB migrations.
The inconsistency between the configuration and values shown in the configuration page, and the actual criteria used for the purge is misleading.