When do you go to the hospital?

[made4this]

I feel a little silly asking this, but I have never gone into labor on my own. My first 3 were inductions, but this time I am hoping to go into labor. My question is, if you were gbs+, how did you time it so that you could get the antibiotics, ideally 4 hour's worth, and not a minute more. I don't want to go too early and stall, or too late and end up with baby having to stay. Anyone have experience with this?