The theoretical maximum heart rate you can achieve is 220 minus your age.
Let's say you're 20, then your abs. max heart rate should be 200. "Cardio" is perceived to be on average about 80% of your max heart rate, hence 80% of 200 is 160. the ideal is to achieve a "cardio" level of exercise at least three times a day for 30 mins. "Fat burning" levels are around the 60% level of abs. max heart rate.
Other factors involved here include the time you are at "cardio" level. Many people will get on the bike/treadmill/whatever for 20 mins and may actually NEVER achieve the "cardio" level to really get the aerobic reactions going in your body.
The fitter you are, the faster you will reach your aerobic/cardio level and for fit people may take up to 5-10 mins....so if you're not fit, it takes longer...so if you thought your 20 mins on the bike was cutting it, think again. All you did was a glorified warm-up.
Everyone is different, so these figures are give and take, but you get the gist of what I am saying. For instance. I am really fit, my resting heart rate in the morning is close to 40. However, 5 mins into running a 6:30min/mile pace and my heart rate is close to my theoretical max...
The true test of cardio fitness is to see how quickly you recover from strenuous exercise - see how quickly your heart rate will drop in two mins right after strenuous exercise - the faster your drop the fitter you supposedly are. I will drop from about 170bpm to about 110 in two mins. Two mins thereafter I will be closer to about 80.
The "perception" levels mentioned in previous postings are also good, however, most of us are inherently lazy and will peg ourselves at a higher perceived rate than reality dictates.
Another interesting fact is this - try running on 10 different types of treadmills and you will get 10 different sets of stats even if you run the same time/speed on all of them. Use those stats as a rough guide.
Oh, and BTW, if that heart rate indicator is blank when you check your heart rate, don't worry, you haven't died...it's just not working....
PS - USTriathlete on RJ is a good person to find out more info on this topic! He is probably the best triathlete on RJ and a personal trainer.