Start with 10mins and if you want a cardio session for variety, build up to 15-20mins.
Make sure the pedal length is good for you - when your leg is at fullest reach when biking there should be a slight bend at the knee. If the length is too short, your knee will stay too bent and the muscles won't work effectively, if it's too long, you risk over-extending the knee and you can't power the stroke all the way round.
Both the exercise bike and recumbent cycle have the same programmes and you can do straightforward pedalling or program in hill sessions, races etc.
If you ever have a bad back use the recumbent bike as the back is in a straighter position which will be more comfortable.