Since ontologies are loaded asynchronously, the end of the loading process should be made available through DBus. I can provide the patch for Nepomuk Core too if needed.
Testing
1) Log into a session
2) Open a terminal and type
dbus-monitor "type='signal',interface='org.kde.nepomuk.OntologyManager',member='ontologyLoadingFinished'"
3) Wait for the ontologies to finish loading, you should see dbus-monitor reporting the signal