The childbirth educator I worked with last fall tells her classes that at the moment that they think they couldn't possibly stand getting up, putting their shoes on and getting in the car for the trip to the hospital is about the time they should go.
When I am physically present with a doula client (I am terrible at judging over the phone), step one is when I see that she gets into a regular labor pattern with contractions that feel very intense and she has to really focus on that last 45sec-1 min. Some clients want to go in then, and usually I find that they get to the hospital at about 3-5 cm when we go. Step two would be to watch for an uptick in intensity after that, including a number of possible signs like increase in duration, decrease in spacing, vomiting, shaking, squirming and trying to run away (though some moms will do this from the beginning), turning deeply inward and acting almost like a sleepwalker, panicking, grunting at the peak of contractions, etc. When we go at that point, moms are usually somewhere between 7 and 8 cm, and some moms will be complete.
I am a lot more conservative with second time moms than first time moms. With a first-time mom you almost always have a little extra time with pushing. I have seen more than a few second-time moms push out their babies in 2 or 3 pushes. For moms who are on baby 3,4,5,6,7+, we will usually go to the hospital once labor is in a regular pattern because it can often be quite fast.
Hope this helps!
Stacia -- intrepid mama, midwife, and doula. Changing the world one 'zine at a time.