i would suggest no more than 90 minutes a week so as to prevent muscle wasting. that is a general figure, and it's likely to be something that you can start at, and go from there. i've been lifting for 8 years (3 years since puberty ended), and i have found that my body responds best (in terms of allowing me to lean mass - no fat at all) to mountain biking.
depending on the type of cardio you do, the time alloted will change. for instance, with cycling, since it is one of the most anabolic cardio workouts so to speak (gets those thighs burning, while not really taking to much breath out of you), i can afford to do around 45 minutes 3 to 4 times a week.
never run...running is the quickest way to take an inch of your arms under the sun.
stairmaster is inbetween (in that it will make you small if you aren't careful, but it isn't nearly as catabolic as running)... when doing something like this, i'll limit it to 45 minutes twice a week.
try to do at least 30-40 mintues of cardio per session. much less, and you aren't really delving into fat stores so much as glycogen stores and whatnot.
finding the correct formula for yourself takes time, but when done properly, it is highly effective overall.
i've gotten some of my best gains (in strength and lbm) while doing around 90 minutes of cycling twice a week (outdoor of course, indoor is torture).
just make sure to eat a lot...because cardio will stimulate your metabolism, and you're going to burn calories while in action, so eat all day, and limit fat.
that's what has worked for me. this type of routine has made it possible to eat almost indefinately, and not gain fat.
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