Art did a regular 5 hour live show starting at 11PM Pacific Time in the 1990s until he retired the first time.
Since it was so late on the east coast many stations there didn't carry the final (fifth) hour though.
When he returned for full time again in early 2001, the show was cut down to 4 hours and started an hour earlier.
When Noory took over in 2003 when Art retired again, the show remained 4hours as it is today.