What is the average angle of attack of GA airplanes during takeoff?
Depends on how slow you are going at rotation, and how aggressively you rotate. 
Stalling AOA of most GA wings is somewhere in the region of 14-16 degrees and on takeoff it shouldn't get above 10ish to have adequate margin.  

The issues over taking off with frost is due to the frost reducing stalling AOA to an unnaturally low angle, say 9 degrees, so that the pilot gets a surprise during a normal pitch up.