It depends on the number of passes required and how big an area. A few not to deep scratches? 15 minutes or so. The entire car with 2 or 3 passes? As much as 8 to 10 hours with a PC 7424.

I ended up driving about 5 cars behind a construction truck that kept dropping rocks. I now have not only a big scratch in my front bumper but actual tiny chunks taken out of the bumper. The scratched/pitted spot is about 1 1/2 inches round, almost like a large rock hit me. My hood has a chip about 2 mm in size from a few weeks ago.

I also had an almost zero miles per hour brush against a steel pole at the car wash which put a couple of small black scratches in the side of my bumper.

At 10,000 miles I am really bummed out.