i think it is enough but if you are out of shape you should consider 45 minutes
But it's grossly inadequate if you're training for the Olympics.
Really, now, when you ask the question, you need to identify your current condition (e.g. sedentary and out of shape) and your goal (e.g. to be a track star).
You also should know that the intensity and focus of the exercise itself changes things a lot. If you ride your bike up a steep incline for 30 minutes, the amount of energy you burn off is not even in the same universe as the amount you'd burn coasting downhill.
Likewise, while bike riding is a good aerobic exercise if it's done right, it will neglect certain muscle groups while overbuilding others. Take a look at strong amateur bike riders' bodies -- you'll see the effect of this quite strikingly. They tend to have very well developed thighs and rumps, while relatively less developed chests and arms. If you want a more balanced exercise that does not impact your joints, consider swimming.
