An egg only survives for up to 24 hours after it’s been released. Once the egg dies, it cannot be fertilised, and you won’t be able to get pregnant that cycle.
The fertile window is longer than the 24 hours that the egg survives as sperm can live for up for 5 days after sex. The fertile window is, therefore, usually 6 days long.
If you are trying to get pregnant, ovulation tests can identify this fertile window during each cycle but must not be used for contraceptive use.