As with most simple questions, there is no simple answer!
It really depends on the intensity of the cardio, and your goals. If you are aiming to loose weight, and do moderate intensity cardio (i.e. where you are working at 65% of your max heart rate) you can do a lot before you do 'too much'. If you are aiming to improve your cardiovascular system and workout up in the 80-85% (or higher) you can easily do too much, and 'overtrain'.
A few years ago I was in your current position. The best advice I was given....'buy a heart rate monitor'. I was able to monitor my 'real' effort - was I working hard, or just thinking I was working hard.
Last edited by gerrymcd; 12-20-2011 at 11:01 PM.