I would like to try doing 2 hour cardio everyday for a week. Maybe 3 hour if I can. I was wondering if there would be any side effects?
My goal is to increase calory deficit to 2000-3000 daily. My daily intake will be around 1500 calory.
I don't think you'll be able to survive for very long doing 3 hours of running (as an example) eating 1500 calories a day. 2,000 - 3,000 calories per day of deficit (depending on your needs) is really hard.
I'm assuming this is for weight loss, and I think you'll have much better results following the tried and true mechanisms for fat loss and body composition.
So while there's nothing necessarily wrong with multiple hours a day of cardiovascular exercise (plenty of cyclists, distance runners, distance swimmers and other athletes fall into this category), you need a diet capable of maintaining that sort of output.
Aim for a deficit in the neighborhood of 3,500 calories per week. That's (very) roughly 1 pound of fat. It takes months and years to get fat, it takes months and years to get thin. And when you get there, you want to have changed your life in a sustainable way. Crash dieting isn't maintainable, and you'll be on a yo-yo of weight loss and weight gain.
It can be done but it's not sustainable, particularly on the amount of calories consumed. 2 x 90 minute sessions would be the better alternative and as mentioned after you've built up to that point and it should be in the final stages of your " cycle "
If the cardio is that intense you will just burn muscle however and again, particularly for the mount of calories you're talking about having.
Slow and steady. There is no quick fix. But when the time comes go nuts on the workouts as in go hard
I'll add a slightly different take on the other answer:
Yes, you can do 2 or 3 hours of cardio in a day. But you should build up to it. You'll find that many cyclists and runners will do a longer workout or two on the weekend, but maybe just an hour on weeknights at a more brisk pace. A rest day isn't a bad idea either.
The important thing that you've recognized is that it takes a lot of time to burn calories through exercise.
Weight loss and fitness go hand in hand, but a healthy diet is the most efficient way to control your calorie intake / deficit. Exercise burns calories but also tends to make you more hungry.
In my opinion, the most important thing you can do is start making your own meals. If you eat out all the time, it is very hard to know how things were made or what they really contain. Start with meals made at home, preferably with fresh ingredients. Get into that habit for a few weeks until it's automatic. You can start analyzing calories more after that. I'm betting you'll find improvement already though.
Exercise is still important and you should absolutely keep it up. But what you put in your mouth will have the most effect on your daily deficit or gain.
As Eric Kaufman pointed out, a slow, steady, manageable deficit is most likely to keep the weight off long-term.