How long?

I've read (and been told by my trainer) that it takes about 20 minutes of cardio to get your liver to use up it's glycogen store before your body moves on to burning stores of fat in the body. If you weight train first, you will empty your glycogen in your liver before you even start your cardio and it can be more beneficial.

You'll hit the fat burning process after twenty to thirty minutes, but use your diet to lose weight. It'll make life easier.

Ive been told you need to exersise for 20 minutes befrore you start burning your bodies fat storages, before that you are just using the carbs you have consumed. ( but any calories burned is a good thing! ) I use my treadmill a lot too, & a runner told me to build up my endurance pick a comfortable pace to start from, then after a couple minutes try doing 1-2 mph faster for 30 seconds to a minute, to challenge your cardio fitness, if you arent out of breath then try a little faster. It all depends on your fitness level. but gradually increase your faster sets & make your slower rest periods shorter.