Norris harps are built to be robust & reliable professional instruments and they should not require a great deal of regular maintenance. However, if you play a good deal, it is wise to have your harp checked over and serviced every 2 or 3 years if possible.
A general harp service can usually be completed within 3 hours and would normally include ...
... renewing the bass wire strings
... checking, regulating & tuning the semitone levers
... cleaning up the whole harp including the repair of minor dents & scratches to the varnish