i have no idea how many times i have changed my brushes. One little idea that i read on here, with power going to the router try tapping it lightly with a hammer. If it tries to start then its the brushes. I get mine on ebay and generally have about 3-4 spare sets on hand.
The first time they were ground down to about .35 of an inch and and another time they were below .5. Loosing your brushes during cutting can cause you to destroy the board you are working on.