Answer : That really depends on how much time and effort you're willing to put into learning interior design. If you're just interested in the basics, then it shouldn't take too long to learn the ropes. However, if you want to become a professional interior designer, then it will take significantly longer to gain the necessary skills and knowledge.
If you're starting from scratch, then a good place to begin is by reading some books or online articles about basic interior design principles. Once you have a general understanding of things like color theory, scale and proportion, Furniture placement etc., you can start experimenting with these concepts in your own home.