Can anyone recommend the time duration and the time steps to set, in a transient DES turbulence model setup for analyzing generic car body shape, such as the ahmed model. thank u, any advice would be helpful.

Your time step should be such that your Courant number ~ 1. Since Courant number is also a function of mesh, and your mesh resolution needs to be fine enough to model the large eddies, your time step will somehow be dependent on your mesh size. Your time step also needs to be small enough for temporal resolution of the eddies. I think you can resolve some of these issues by using the SAS turbulence model instead.

Not sure what you would use for time duration. I suppose it depends on why you are doing the analysis.