This fitness guru guy I know, told me that when I do cardio I should do 45-60min since the body doesn't start buring fat until you reach about 30-40min of workout. Is this true? should I only stick to the cybex arc trainer for 45-60min then do 15min of light weights - i am trying to loose weight and get that lean look. Or is it better to do 30min of arc trainer then another 30 on the treadmil then weights ?

first off, you should ALWAYS lift weights first.
second, 15 mins isn't enough weight training to be worthwhile.
third, I'd split up your routine and do weights 3x a week, cardio 3x a week, alternating them.

saying that fat burning doesn't start until 40 mins into cardio is too general a statement to be true.
The body can burn fat as long as its aerobic exercise. But you need to increase muscle mass with weights, because muscle burns calories. if you diet and exercise yourself into looking like a toothpick, you'll have to eat less than a hamster to maintain that weight.

if you put on muscle, and shed some fat, you'll be able to eat more, because you'll have to eat just to maintain the muscle mass, in addition to eating for energy to get you through daily activities.

45mins of cardio is god's plenty. 20mins of intense, interval style cardio is even sufficient, but it does vary from person to person, as well as beign dependent on your goals and your current level of fitness.

20 mins of intense cardio affects a morbidly obese person far more than an athlete. your magic cardio interval might be 25mins.

If you were doing 45 mins, that's slow, steady state cardio, and that kind is best done first thing in the morning on an empty stomach. A 3 mile brisk walk would qualify...even a few little sprints or jogging a couple blocks in the middle.

for evening cardio, 20-30 mins of higher intensity cardio, or intervals of varying intensity is a better system.

Agree with Malkore on splitting the routines, and interval training.

You often hear the 45 minute claim of to get in the fat burning zone, which is a pretty blanket claim. If you do very low intensity you could be in a fat burning zone very soon, as you aren't reaching an aerobic zone yet. This seems counter intuitive but i've participated in some nutrition and exercise tests (as a guinea pig), and University College Hospital in London, that confirm this. On the other hand you wont burn many calories so its not much use.

Your fat metabolism can depend on several things like your glycogen levels, and even things like taking caffeine 30 min before a workout can help kickstart your fat metabolism earlier.
A morning workout will also get you there sooner, as mentioned above.

I wouldnt worry too much about which machine you are using, just keep it challening and use one you like using, or even get outside and go for a jog and some sprints if you can.
Also when doing your weight make sure that you do them at the right intensity at around 8-12 reps a set, rather than jumping from machine to machine. Try having a go at some free weight exercises if you can, but make sure you get someone to show you how to perform the exercise correctly.

thanks guys. James, so you burn fat with low intensity and calories with high? seems strange.. don't you burn fat when you burn calories ? Basically, I am 5'6" at 185lbs. I don't look obese but I think I am a bit chunky and would love to get that chiseled-soccer player look. Basically toned muscles, not big since I am short and anyone who is bulked up when they are short looks awful.

Fat oxidation during exercise is irrelevant when compared to total calorie expenditure. That's what matters.

Best and most simple way of putting it.