Hello,, don't worry it doesn't injure you, or your sperm doesn't get decay in your body. It's very hard to tell how long a person can store the sperms,, because it depends on how much he is producing a day,, it inturn depends on his,, daily healthy diet,, stress,, activities,,and also smoking or drinking. If enough semen is produced, which has no place to stay in ejacultory duct then it will secrete out knowingly (masturbation) or unknowingly (night fall). So don't worry about that.