In order to apply for your visa you must be entering Turkey within the next 3 months.
Turkey Visa is valid for 6 months (or to be determined by Consular), with single & multiple entry visa options.
It allows its holder to make single or multiple visits and, depending on the nationality and passport type he/she can stay one to three months each time he/she enters into Turkey.
Side note:
A single entry visa is valid for 6 months or less and allows its holder, depending on the nationality and passport type, to stay in Turkey up to three months and to visit the country only one time.
A multiple entry visa is valid for up to 6 months and allows its holder to make multiple visits and, depending on the nationality and passport type he/she can stay up to three months each time he/she enters into Turkey.