I built up a library of dark, bias and flat frames for my CCD camera that I reused many times over. Provided your light and dark images are taken at the same camera temperature and duration then darks and bias images can be reused for a long period. I refreshed my library about every year, not that I needed to but just thought I ought to. Flat frames can be reused provided the focus position remains the same and the camera has not been re-orientated or moved in any other way.
I’ve just moved on to a CMOS camera and am re using the darks and flats. Does any have an opinion about the re use of CMOS calibration frames?