It will take something like a time proportional to the cube of the number of mesh elements or points, per frequency. So proportional to the surface area of the reflector, cubed. For a mathematically generated source (like a plane wave or a spherical wave in the far field, or moderate near field)it won't depend on the source, but if the source is a model which has to be meshed in the same calculation, add those mesh points before cubing.

In addition to the above, in the log file (after finishing the first simulation) you have very valuable information to know the times and try to optimize them by reducing (for example) the meshing of the structure or the number of calculated points.

Dear Petr Polovodov

CST Microwave Studio use Finite Integration Technique ( FIT ) in Time domaine simulation wich is based on FDTD Method in Time domaine, CST use also Transmission Lines Matrix ( TLM ) for others excitation purposes. But it doesn't include MoM Method.

If you want to use The MoM Method to perform your simulation you can use

other commercial simulators which based on MoM Method like : FEKO, Momentum -ADS, ...

For Time requirements scale proportional to the size of structure being analysed, the mesh properties that you would to set up and the required solution frequency. But the most major advantage of MoM over other EM mogeling techniques stems from the fact that it requires only discretization over the structure in question not also the free space surrounding the geometry.

Said Choukri CST uses FEM in the frequency domain, which is not quite MOM, but similar.