The issue you face here is that the multiplication of constants is a constant and therefore has no timing.
A method to measure the timing would be to synthesise the multiplier with 2 inputs. Then perform a gate level sim including the SDF timing information. Taking both inputs from 0, to the fixed values. In a waveform editor you should be able to see ripple on the output of the multiplier. Measure the time from new input to settled output.
Remembering that Silicon timing is quite variable, You might get fast or slow (or centred) Silicon. Temperature will also effect the timings. You will need to run the sim with at least two corners Max and Min. Resulting in a max and min time to transition from (0 input to) fixed input to stable result.
Also realise that the timing used in this method is from 0, it will be different with different starting conditions.
You could try having two LUTs (Look Up Tables) holding the input values 0 and your fixed ones and see what your synth tool does. Likely will optimise to a LUT with 0 and the answer.