I only set a bike up on the trainer when it is not possible to ride.
Then, I set a timer for as long as I think I can stand it and try to tolerate it for that long and no longer. While watching TV, comfortably reclined on the long wheelbase 'bent.
Hermes and AJ describe actual training while I am just putting some time in on a trainer.
__________________
Bacchetta Giro A20, RANS V-Rex, RANS Screamer