it is very critical to answer onto 'how much' denoising is possible.
I think you mean denoising in addition to the denoising, which has been already done during pre- and post-processing prior to display the image(s) by the appropriate modality. To my opinion denoising for example in x-ray images has been done already very intensely due to the everlasting demand on dose savings.
Additional denoising is very special and will extremely depend on the case of interest.
You may for example go through these review papers in order to get an impression on how denoising is done: