If sperm do not fertilize an egg, how soon do they die, and what happens when they die?

Don't die together: Sperm that's put in the vagina don't all die together at one time. Most die in 2-4 days. They are living cells, swimming around, and so every man/woman/couple are different. A woman has varying hormone levels, and thus different mucus & environment, in her body on each day of her cycle, so sperm may live longer/shorter on different days. Dead sperm break up. Some become part of vaginal discharge.