i was buying 6* girls when they showed up and stuck with the 5* event ones for a long while... I've probably spent over 500-800 on refills within the year or so i've been playing....
it's more based on your preference, cause even at lv130+, it takes 4-5 runs of 80-100 stam stages to kill a whole bar... stamina potions only do but so much...
but if you're a free player, SPAM THAT GACHA. The tickets suck....