It doesn't take very long at all. Just get a female and a male sim that are in love, buy a double bed, give them both the Relax interaction, then click on the Sim which isn't active and click Try For Baby. Then, if the try was successful, a musical chime is heard. About eight hours later the woman is sick, then six hours after that she's sick again. About four hours after being sick for the second time she usually changes into maternity wear and is officially pregnant.