I think I ran just one magic food at the mistlands boss, but the fight took quite long as a result (just about 30 minutes). It really depends on your personal preferences, the scenario and whom you play with. If you want to be economical, you can consume just magecap mushroom as well as stuffed mushroom to get enough eitr to cast blood magic spells, and then train your blood magic skill to reduce the eitr cost. So, as is the case with all weapons, as you train your magic skills, you can do more with less eitr.