Used to run a computrainer center for my club. Test that worked best was 40 mins indoors on a rolling course. Basing off that number the workouts went well. Hard to finish but nobody blowing up.

We used 20 min test for a couple of years and it was a disaster. Lots of people failing workouts. Unfortunately, riders get obsessed with the 20 min test and deliberately go easy on the 5 min blowout in order to save energy for the 20. But even the ones who did the blowout still failed. The person who was running the center at the time felt that if you weren't failing you weren't trying hard enough. Refused to consider that everyone's ftp was set too high.

Coach of a current world champion told me that he knows one person for whom 95% of 20 mins was accurate.