I'd say as often as you feel a soft pedal. This question is so variable based on brake fluid, track, braking technique (and obviously car). I haven't run the E9x M3 on the track, but I'd say on average you'd likely be okay doing 3 events before bleeding, assuming those 3 are in one calendar year.
2011 Jerez Black/Fox Red E90 M3 DCT, ZCP, ZCV, ZCW, ZP2, BMW Apps
2004 VW R32 (track car)