It really depends.
I've known people that date several people at once for weeks and weeks and even if they've gone on several dates, but they're still not decided. It's more of a casual thing.
Then I've known people who have gone on three or four dates and it's official.
For me personally, it's when you start hanging outside of a formal date setting. They text you to ask where you are and you spontaneously grab a cup of coffee instead of going through the two-three hour grooming process of picking out an outfit and a place to go. You chat more often, text back and forth, having late night conversations. You just become more.... couple-y.