Is 40 mins a night good on a exercise bike?

Cadence. This is the key to what you want to achieve. This is, in effect, your rpm. You want about 90 rpm as a cruise speed. After that, resistance. I tend to vary my resistance, in order to replicate hills etc. Another good method is to pulse your resistance, alternating between 2 minutes of stiff and soft resistance. The trick is to keep your cadence the same for both, or even to increase cadence considerably when slack resistance. In any event, you are aiming to get your heart rate up and break a sweat. I go to the gym sometimes and note some of the other people using the bikes. They sit there, with their magazines, resistance on minimal barely making 30rpm and I wonder why they are paying their membership, as with that minimal exercise they are not actually doing anything. So use the resistance, use your speed and know that you have had enough of a workout to feel it the next morning. Luck

Yes thats great, keep at it and then up the resistance a little once youv'e become used to what your doing now. Then you'll become stronger and fitter and be able to burn more calories as you work harder.

Personally I ride my bike about 32km every day and I am fit. To make it hard on yourself make sure you doing uphill and such, that way your not riding on a flat plane

That sounds like it would work.