Pulling a vacuum without a good vacuum gauge is an exercise in guesswork. And by good vacuum gauge, I do not mean a standard manifold gauge set where there is 1/2mm at most between 29" and 30" Hg on the dial. That one inch of vacuum is 25,000 microns, and you need to get it below 1000 to boil the water out. How are you going to tell? Additionally, if your vacuum pump oil is old, you won't get as good of a vacuum. I've changed vacuum pump oil in the middle of a job for prophylaxis, but with a meter (which I now have), you know for sure.

It took about four hours to evacuate my car. After the first five minutes, the gauge on the manifold set showed 30", but the micron gauge gave me the real story. It was in the tens of thousands at that point.