Does semen become infertile after ejaculating for any period of time? Or does it ever become infertile, and if so when?
Depends on different factors. Sperm is very sensitive to temperature.
Typically ejacualted outsde the body say into a kleenex or what not it will not live for much more than an hour.
If ejaculated onto skin or into a warm test tube and kept at body temperature it will live for several hours. When ejacualted in the vagina it will live for about 6 hours When sperm enters the female reproductive tract (pass the cervix) it can live for up to 7 days (3-5 days is more typical).
sorry, I meant if the semen reproduced within the body is infertile if semen was recently expelled.
semen can live in the vagina for up to 2 weeks with no problem.