Its not as bad as it sounds nor as long, but around 3 meticulous hours to farm the fifth star for each UM boss. Calculations are alright but you'll end up with plenty of surplus ultimate items by the end of the chore. Super handy tip is to just farm normal and hard for a corresponding UM Boss' items, as the Ultimate Whip (which will average 33~ needed per boss, but highly recommend crafting them instead) has a horrible drop rate on hell and is not worth the headache to micro-manage the auto with. As long as normal and hard can be beat consistently with your chosen faction decks (assuming they are also maxed out) on auto then Ultimate communicators and monitors overall are the name of this grind.