C:\Users\username\appdata\roaming\autodesk\autocad 2013 - English\r19.0\enu\plotters\plot styles which is also %RoamableRootFolder%\support.
Instead of memorizing this path, a quick and easy way to restore the default is to delete the temporary path. This will set the path back to the default location.
For example, if I change my Plot Style Table Search Path to a custom location:
Then, when I am ready to change it back: highlight the path, click Remove (the path becomes blank), then click Ok.
A dialog saying: "...a path was not specified. The default location will be used..." will display. Click Ok and the path is restored to the default.
This can come in handy for a quick test situation or when helping another CAD operator restore a profile path that was inadvertently changed.