I lift 3 times a week and do a least 30 minutes cardio after lifting...an hour if I have time. These are light cardio days. I also do higher intensity interval training 3 times a week for an hour to 1 1/2 hour, again depending on times. It took me a while to work up to that.
Anyone should be able to do at least 30 minutes of cardio, even if you are out of shape. Most people can walk three miles, which should take about 45 minutes. Try 2 miles if that is too much. And that is low intensity.
I HIGHLY reccomend getting a heart monitor and learn how to use it. I posted these links on another thread, but here are a few good things to read and then get that monitor.
I personally think 30 minutes 3xweek isn't enough, but it is better than nothing. 30 minutes 5-6xweek is much better.
http://www.duathlon.com/articles/1460
http://www.trifuel.com/triathlon/tri...ate-000574.php
http://www.lifetimefitness.com/modul...ning_chart.pdf
The last link gives you a great chart to figure out where you should be. Most people work out way to hard and don't understand how cardio can really be effective.