In the winter, after the water temperature falls below 70 degrees, I run mine for 4 hrs every 2 weeks. I do keep chemicals balanced and net out leaves. I have never seen this recommended but concluded I could do so after my pump controller died and nothing really bad happened. Don’t neglect to monitor the chemistry, maintain algicide levels, and remove leaves/debris though.

During the season, you want to turn the water over at least once a day. possibly twice if it is used heavily or you get lots of leaves and trash.

For a 20000 gallon pools with a 2HP pump that flows 80 gallons/minute: 20000/80 = 250 minutes = 4.2 hours.

I don’t think there are any hard fast rules on how long to run the filter. If it looks good, it is good.

If you are now running 8 hour a day a slow pump speed, that may be perfectly reasonable. The big savings was probably reducing your effective filter time in turnovers/day.