Varies a lot, even within the same seating class in the same aircraft. For example, in 737 economy class on any airline that uses those airplanes, the window, middle, and aisle under seat spaces can be different sizes.
The window seat under seat spaces are often oddly shaped because of the curvature of the fuselage. As previously stated, some non-window seats may have some space taken up by in flight entertainment equipment.
There is also variation due to airline policies on how far under the seat stuff put there has to be (based on airline interpretation of how far sticking out is a hazard for someone climbing out in an emergency).