My max spin speed is around 440, average around 350. Never practiced it, it's just sort of increased over time.
If you want to actually get good at the game (Like I'm the one to talk about getting good, right?), practicing spinners is the greatest way to completely waste your practice time. You'll learn to do them well enough by playing normally, and the spinner bonuses don't matter at all on difficult maps. Like, you can literally lose more points from getting one 100 near the end of a song than you'd get from all the spinners in that song together. It's just not worth it.
However, if you want to disregard actually learning the game just to spin fast, then go ahead and practice by spinning your cursor in circles.