Everyone is different. Some wait till after 12-13 weeks because at that point the chances of miscarriage goes down significantly.
Some will wait till 26-28 weeks when a premature baby has a greater chance of survival out of the womb. Some wait till they are 37 weeks which is full term, and some never tell unless a family or friend asks. Or you have people like me, who tell the second they find out! lol. I took an EARLY test, and we were too excited to keep it in! We only told family and close friends though, and by 6-7 months there was no hiding it!