Start off with 20 bubbles/minute.
Using the chart on this page: http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm
cross reference your pH (across the top) with your KH (across the left side) to find out the CO2 concentration in your tank.
If it's low, increase the number of bubbles per minute slightly. Test again the next day.
If it's high, decrease the number of bubbles/minute slightly. Test again the next day.
Other important bits to consider -
1. Have you ever added anything to your water to change the pH, GH or KH? If so, what?
2. Take a clean drinking glass of water from your tap, and let it set overnight. From this sample test the pH, GH and KH of your tap water. (you're letting it set to off-gas any nitrogen, oxygen and CO2 that may have built up in the sample).
3. Take samples of the tank water's pH, GH and KH. You can compare these to the numbers from your tap to see if anything in your tank is affecting hardness of your water. If they are drastically different, ask questions.