How to retain all builds/artifacts for unlimited days in Teamcity 8.1.3

That should be the default behavior of TeamCity. See: http://confluence.jetbrains.com/display/TCD8/Clean-Up

The default behaviour has been changed. How do I redo that?

1) Contact the TeamCity administrator.
2) Ready to control whether you have the right to edit the cleanup rules (but I suspect not).
3) Save your artifacts anywhere else ....