IMO..when doing am cardio in order to burn through remaining glycogen stores to get to fat you need to go longer at least 35-45 minutes. If doing it post workout after training big bodyparts like back and legs only 20-25 minutes, if the bodypart has been trained properly.
On some days I do just cardio first thing in the morning on an empty stomach (still working on losing fat). On other days I do a bit of cardio to warm up, then do weights and then more cardio to finish off.
I'm also trying to increase my fitness levels so I do 25kms on a rowing machine on the non-weights days, and 10kms on weights days.
I usually use an elliptical trainer/ski machine for about 20 minutes to warm up.