The Honest Answer is; It depends!

What are you hoping to get out of bike training;

Do you do it purely to improve leg strength for fellrunning?
Do you compete in TT or Triathlon, MTB etc?

There are some general tips;

No session above 40-50mins (if you can go longer than this then you have a seriously high boredom threshold!).

Dont just go flat out for 20 minutes (numpty advice!)

Warm up properly, cool down properly.

Make use of the strengths of a turbo trainer (controlled resistance, safe environment) to devise your own program e.g

If you are after pushing up your lactate threshold then do some short intervals in high gears, followed by higher cadence recoveries.

If you want to improve race efficiency, use it to raise your cadence to 85rpm plus (increased cadence always feels wierd at first but soon becomes more natural. Insufficient cadence is the main fault of all amateur competitors and it reduces endurance ability, as well as affecting your run performance if you are doing triathlon).

Hope this helps; give us some more info about yourself if you want anything more detailed.