The length of time it will take you to obtain a high school diploma has several determining factors. Some of these would be;
your local area education standards
the amount of high school credits already earned
how much time and effort you put into accomplishing your goal
My advice would be to check out attending an accredited, top quality online high school. Many of them are free depending what country, state, city you live in.
Here is a link to one such educational organization.