How much is alpha induced angle?
The induced angle primarily depends upon the following:


Coefficient of lift
Aspect Ratio (AR)
The correction factor $\tau$ (needs to be accounted for if the wing platform is not an ellipse. If the wing platform is an ellipse, then $\tau=0$)


All three are related the with the following equation:
$$\alpha_i[\text{rad}]=\frac{C_z}{\pi AR}(1+\tau)$$

The table below provides the values of $\tau$ for a rectangular wing.



You can use these values and NACA Report 824 to calculate the actual induced angle of attack for a rectangular wing for any possible airfoil.

For instance, using $\text{NACA} ~65_1-412$ airfoil at $\alpha_{\text{geometric}}=8^{\circ}$ and $AR=9$ it follows that $\alpha_i=0.05~\text{rad}\approx 2.86^{\circ}$.

It varies greatly in relation to the angle of attack, as low as $1^{\circ}$ in high-speed flight with low angle of attack and as high as $3^{\circ}-4^{\circ}$ at low-speed flight for a rectangular wing aircraft.