xan021,
Wow, that's a tough one. Much of it depends on the particular product, or product/pad combination you are using. A lot also depends on your level of expertise with the rotary. Generally speaking, a slower speed will keep a beginner out of more trouble. Although, using a rotary at too slow a speed with some products/pad combos can create problems.
Anyhow, I would say that most polishes can be used somewhere between the range of 1000 to 1800 RPM's.
My advise is to get someone with some experience to show you how to use the buffer, and/or practice on some "junk"