system.teamcity.build.checkoutDir.expireHours not working

Hello, i have multiple projects set up with the above System Property, but their workspaces are still getting stale after ~2 weeks and enforcing a clean sync.
Am i setting this property incorrectly?  Everywhere i've searched for says to set "system.teamcity.build.checkoutDir.expireHours" to never in the project configuration parameters.

I would like to set this variable via the web interface (via 'system.teamcity.build.checkoutDir.expireHours' Build Configuration property) instead of in the text file on each agent's machine.


Thanks.
Delfino