I take flats, made up of about 50 averaged images, each time something in the optical train changes, like moving the camera or a new blob of dust appears or I use the camera at a different set tempetature. I redo the flats if the season changes but nothing else has changed. However apart from that I keep using the same ones. I’ve experimented and can’t find any decernable improvement if I take flats every time I slightly tweak the focus.
British Astronomical Association
Supporting amateur astronomers since 1890