If you go up 100 points in less than a year then you're fine, don't even worry about it. Instead think about where you can make improvements and your rating will take care of itself.
I want to get to 1500! Maybe even higher
i have been playing chess for the past 3 years, i study tactics every day, i study end games every day.i anlyze my games.with chess base every single day.but my rating is 715.i think i am mentaly ill.

You are right, there are many factors. However, you have made it to 1200 and in my experience players who start below 1200 tend to hit a wall for awhile at that rank and, after some study or many many games, will eventually push past that. I did.
Anyway, it is nearly impossible to say how quickly you will be able to reach this rank or that rank as it depends solely upon you. Factors such as how quickly you catch on to ideas, how well you use ideas, amount of time used for playing/studying, amount of talent, patience, etc etc etc, all contribute to how much time is spent.
Putting all of that aside for now, I am a firm believer that hard work does pay off. But we as human beings, and especially most Americans, want things now, as in yesterday. If you can push past how long it will take and shoot to improve regardless of the amount of time and continue trying hard to improve in the correct ways, then I can guarantee that you will make it up to and potentially past 1500.
How much time is solely up to you.
How long will it take me to get to 1500?
Better yet, how long does it take the average person?
Of course I know it depends on many factors. But say I play a few games a day like 3 or 4. Do the 3 tactic puzzles everyday. How long?
It took me like 2 months to get from 1000ish to 1200ish and I think that is a long time. Now I am stuck in this 1200ish spot and not moving up. It is frustrating to know that there are scrawny little 12 year old kids that can play better than me.
Am I mentally uncapable of making it up to a higher level? Do I level off? If so when do people level off where they can't get any higher. If at the rate I'm going is it impossible for me to become a grand master. I want to become a grand master eventually, but at this speed it seems I will never get there.
I am an ambitious person, but want to know whether this is a realistic goal or not.