I do a 75 minutes recovery once or twice a week on the trainer. I keep my wattage at 56% of my threshold or lower. So basicly I ride like a grandma for 75 minutes.
Here is the actual workout description from
TrainingPeaks.
Bike 1:25 ERG38
1.25 hours, just easy and riding like Grandma. No HR's over 56% of your Threshold watts.