When does automatic clean of checkout directory occur? Follow

When does the automatic work directory cleanup as described here actually occur? Does it only happen when the free disk space is below the disk space threshold (default of 3GB)? I've set the 'system.teamcity.agent.build.checkoutDir.expireHours' agent property in the buildAgent.properties file for one of my agents to 72 hours, and yet I'm not seeing work directories older than 3 days being automatically cleaned up. The documentation says "The directory cleaning is performed in background and can be paused by consequent builds", but I haven't seen anything actually be cleaned up in the last couple of days. Am I missing something?