Most of the sites I have seen recommend increasing the heart rate for a short period, followed by a longer recovery period. How many RPMs that is depends on what shape you are in. So your best bet is to get a good heart monitor and go by that. Short intense bursts of energy followed by recovery are the most efficient at burning calories. A simple template for interval training is:
Time Intensity
10 seconds hard
60 seconds recover
15 seconds hard
60 seconds recover
20 seconds hard
60 seconds recover
20 seconds hard
Rest for 2 minutes and repeat. Then complete a warm down for 10 minutes.
Here is a different stationary bike workout that might work for you:
Using the manual program, enter your workout time as 30 minutes and choose appropriate level
5 Minutes: Cycle at a comfortable pace. PE=5.
5 Minutes: Raise level several increments and decrease a level every 30 seconds. PE=6-8.
5 Minutes: Cycle at comfortable pace. PE=5.
5 Minutes: Raise level higher than before and decrease a level every 30 seconds. PE=6-8
5 Minutes: Cycle at comfortable pace. PE=5.
5 Minutes: At current level, increase a level each minute. PE=7-8.
Calories Burned: 245 (based on 140 lb person)