With Sprint edges I get 2 good runs. If I am at a big race I will cut the com betweens runs. For Club racing I leave the motor alone for 2 runs.
For club racing I usually use the Enduro Edge brushes. I get about 6 runs out of those (2 runs/cut com,2 runs/cut com,2 runs/cut com) before that get too short or burned up.
With the MR springs you wear the brush faster than with the stock stiff spring. Keep an eye on your brush length. Sprint will get short in 2 runs, enduros, as I said before in 6 runs.
What I run: Schumacher Mi5/Associated RC10R5.1/Associated RC12R5.2/Futaba/HobbyWing/Team EA Motorsports/BSR Racing
Where I run: Florida Indoor R/C Complex/Thunder Racing/Florida On Road State Series