It has happened suddenly, but methinks they have left.
I put fresh nectar out last Fri or Sat and we still had our 4 or 5 birds fighting over the feeder. Monday, no activity, save one that came early morning while I was drinking coffee. It stayed for about a minute, and then flew off. Haven't seen one since. Maybe it was one for the road.
I'll wait another day or so, and then call it a season.
Dave