I used to do my cardio on its own, either in the morning or evening but without lifting. Now i do 20 to 30 minutes immediately following my weight training. The reasoning behind this is right after i finish lifting my heart rate is really high (I lift hard and intense). So with the heart rate being high throughout my workout, it is already started and all i have to do is jump on a bike or treadmill and just cruise pretty easily to keep the HR up.
ANother tip is not to count your warmup or cool down as part of your cardio. If you want to do 30 minutes then do 5 min warm up 30 min of cardio and 5 min of cooldown. Its all about keeping your HR in the zone and warming up and cooling down ususally isnt in that zone.