Speed of spindle

What should the speed of the dewalt spindles be? I can’t really find a good set of rules on this.

I’m using 1 for plastics with single flute bit,
2 and 3 for slow feed rate with tiny bits, including engraving bit,
3.5 for V bits,
4 for faster feed rates with 1/8 and higher diameter bits.
Don’t try 5, kills your Dewalt brushes very quick.

So far I’ve only used 2 (or 2.5) on MDF, and 5 for aluminum. Like @AlanDavis said, higher speeds for long periods of time aren’t good, but at least you can get replacement brushes on Amazon for $17 US (http://smile.amazon.com/gp/product/B00X1LF708?psc=1&redirect=true&ref_=oh_aui_detailpage_o05_s00).

Actually, $4.97 each, I received last month .