In the United States, from the first date you have publicly disclosed or sold your invention, or even offered it for sale, you have one year to file an application for a patent. However, for very technical reasons and also if you would like to obtain patents in other countries besides the US, it is very desirable to have a patent application filed prior to any public disclosure or offer for sale.
If you have not publicly disclosed or offered your invention for sale, and if no one files a patent application on the same idea before you, you can wait as long as you like to file your patent application.