At what angle of deflections do rudders become inefficient?
The only time you should be using full rudder is when doing a side slip - such as turning the yoke for a left turn but pressing right rudder, so you are descending and looking to your left as the plane goes forward tracking down the runway direction.  Normal coordinated turns, you would not use anywhere near that much rudder while turning.

I just found a NASA Test Report on my plane, a Cessna Cardinal, that lists control deflection amounts as a reference for a light general aviation airplane:

Angle of attack,           -4° - +20° ± 0.5°

Sideslip,                 -15° - +15° ± 0.5°

Pitch angle,              -15° - +15° ± 0.5°

Roll angle,               -75° - +75° ± 0.5°

Rudder deflection,        -24° - +24° ± 0.5°

Stabilator deflection,    -20° - + 5° ± 0.5°

Right aileron deflection, -20° - +20° ± 0.5°

Left aileron deflection,  -20° - +20° ± 0.5°