It depends. Sometimes I buy expansions as needed. Often times I use them for stamina refils to farm certain things needed. Sometimes I do a 5 gem pull and regret it. Once in a while I go for a period of time not using them at all and save up for a 10+1 pull.

Most of them do go for stamina refils tho. Right now I'm using them to refil stamina and farm presents. I have a weird desire to max all the girls, even ones I don't use. A waste perhaps but oh well.