How long will it take to increase 200 points?

Obviously this is a "how long is a piece of string" kind of question. Key variables might be:

1. Your current strength
2. Your age 
3. How much time you have for practice 
4. The quality of the practice
5. How much you had practiced to reach your current standard

Still, it is worth thinking about if one hopes to set meaningful goals. And it goes without saying that your is merely one metric of what might constitute improvement.

Well, I have certainly watched my blitz rating drop by 200 points in a few hours...

To improve by that much in one month is incredible, but to do it in 10 years cant be considered good.

Obviously a critical factor is the format. Blitz and bullet improvement will follow a very different pattern to classical or rapid chess.