Is there a way to Determine the amount of samples needed for a good still image before the actual render?
Yes. Render a small region(ctrl+b, ctrl+alt+b to clear) at full resolution, set samples to something like 99999 and wait for it to be free of noise, see how many samples it takes. You should check the places with most indirect lighting as they take longest to clear. You could experiment with denoising the same way. Obviously it should be either smaller than the tile size or set to progressive refine.
Could also use image stacking technique to get more samples rendered for the whole image, or selected areas that didn’t clear out good enough the first time.
Thanks guys i’ll try it out.