Cardiovascular exercise

The American College of Sports Medicine (ACSM) reccomends that to get the benefits from cardiovascular exercise you should:
Train 3X per week.
At 55-90% of your maximum heartrate.
For 20-60 mins in a single session or cumulative.
The type of exercise should involve a number of large muscle groups.

Ideally everyone would perform a maximum heart rate test in a physiology laboratory to find their true maximum. This is because the methods used to estimate maximum heart rate can be inaccurate. In my personal experience I have been running with a 40 year old female whose theoretical maximum heart rate was 180 and whilst running it reached 200!

However if you do wish to estimate your max Heart rate the simplest method is 220-your age.

Another way is using Rating of Perceived Exertion. The Borg scale is shown below:
6
7 very very light
8
9 very light
10
11 fairly light
12
13 somewhat hard
14
15 hard
16
17 very hard
18
19 very very hard
20

This scale has been shown to correlate well with heart rate where the number on the scale x 10 is close to actual heart rate. i.e. when someone feels they are working hard (level 15) their heart rate will be about 150.

There are other indicators that can help to let you know you're working at the right intensity.
You're obviously working hard if you're getting hot and sweaty. Also breathing getting heavier is a good indicator. However if you're struggling to talk you're working too hard.

Although the recommendtion is to work between 55-90% max HR it is often found that working between 65-85% is most effective.