The life expectancy of sperm depends on the environment where the sperm is deposited after ejaculation.
At room temperature (75 degrees Fahrenheit or 25 degrees Celcius) without dehydration the sperm can live between 4 and 6 hours. The semen (viscous, whitish secretion of the male reproductive organs, containing spermatozoa and serving as their transporting medium) is over 85% water. So then if the semen loses water (dehydration) this material begins to crystallize; all the while the sperm cells also undergo water loss. Therefore, in ordinary dry air at room temperature when dehydration is an added factor, human sperm can live maybe 1 to 3 hours.
A good 'rule of thumb' is; if the semen is still wet there are probably live sperm.
Sperm's lifespan increases once it enters the female body. Sperm usually survive three to five days when ejaculated into a woman's vagina where the cervical mucus helps sustain sperm searching for an egg to fertilize. Sperm may survive in the Fallopian tubes for as long as 3-7 days. This means that fertilization can occur even if the sperm are deposited as much as a full week prior to ovulation. If a woman's cervical mucus is not very hospitable to sperm, lifespan of sperm may be shorter.
Roger W. Harms (2008). "How long do sperm live after ejaculation?". Mayo Clinic. Mayo Foundation for Medical Education and Research.
Lee Ann Hamilton, Sec Talk and David Salafsky, health educators at The University of Arizona Campus Health Service.
Frederick Sweet (1999). "Re: How long can human sperm live outside the body?". MadSci Network.