The length of an arc is given by the formula:
\[
L = \frac{\theta}{360^\circ} \times 2 \pi R,
\]
where \( \theta \) is the central angle and \( R \) is the radius of the circle.
Thus, the length of the arc \( AB \) is \( \boxed{\frac{2 \pi R \theta}{360}} \).