I can only listen to Onward to Golgotha like, twice a week, and often less than that. That level of intensity affects me too much if I expose myself to it over and over in a small time span. Same with Reign in Blood (and most of the other death metal I listen to on a regular basis; Suffocation, Death, Morbid Angel); just enough is almost too much.
To keep the music from becoming overwhelming, I space a lot of Ravelle, Debussey, Stravinsky, and Rachmoninov, and miscellaneous Segovia in between the metal. It helps me to keep the perspective that I prefer.
Now Mercyful Fate, Burzum, Inquisition, Manilla Road, and Celtic Frost have albums that I can listen to every day, but they are not so psychologically demanding as the death metal albums. Maybe if I had more time to just sit and listen and do nothing else, I could absorb more of the DM in a shorter period of time but I don't want that level of intensity to become my norm. Slow, quiet, meditative, and fluid music must come in between the DM, and that makes the experience all the more extreme when I do decide to throw on some Deicide.