Aerodynamics of cycling behind a bus

The zero benefit will theoretically only appear in infinity. In practice, it will of-course be non-measurable in a finite distance. The wake of a bluff body (like a bus) will consist of a recirculation zone, where the Reynolds-averaged streamlines are closed and the flow is actually reversed (in the coordinate system fixed with the bus) in some volume, and the rest where the flow is only slower. The benefit will be decreasing the most at the boundary of the recirculation zone. It will be relatively constant within the zone and it will be slowly decreasing in the wake farther behind. The length of the recirculation zone will be comparable to the lateral dimensions of the bus.

I do not have time nor resources for a proper literature review at the moment but I recommend looking at Figures 3 and 4 in Longa et al. JFM 866, pp. 791, https://doi.org/10.1017/jfm.2019.92 that show typical wakes for two lorry-like bodies.

(© 2019 Cambridge University Press, CC-BY 4.0)

The figures show that the region of decreased wind velocity can reach far behind the depicted region but is the strongest in a small blue region with dimensions comparable to the lateral dimensions of the vehicle, where the U velocity component is actually reversed. That means that the bus traveling in the still air effectively sucks the particles in that region towards its rear wall. The flow in that region can be complicated, the paper itself is about that (bistability etc.). Be aware that the details strongly depend on the shape of the rear of the body.

For a cyclist we have to consider the dimensions of the cyclist too because the low-momentum region is becoming lower and lower, see Fig. 4a. That means that it is difficult to predict the drag of the cyclist in that complicated region. It would be a difficult simulation. But we can still look where the flow is fast and where it is slow.