I've read from 3-7 days. Many factors are involved and will effect how long sperm can live in the vagina. So that leaves a very teenie tiny uncertain window during your cycle that you might not get pregnant.
It's a wiser option to use some sort of birth control - whether it be a barrier method (condoms) or a hormonal method (the pill, patch or shot)
sperm starts to die when its hit air. it never goes into your system it only goes into a cavitey (simply put ) unless you get pregnant then one sperm inters your overyHow long does sperm stay in your body?
Sperm can live for a few hours in the open air, several hours on other things such as on clothes, towels and under nails etc. If you had unprotected sex up to 4 days prior to ovulation there is always a chance of pregnancy. The stats show a few days prior but there have been reported cases longer so this is why I say 4 days. Never have unprotected sex and you eliminate the chances greatly but keep in mind no birth control is 100%.