Sleep on beds or sofas to speed up time by 10x (oh btw there’s also a day night cycle) Ayume starts off with a very small belly capacity. To increase max capacity simply keep her belly full throughout the day. Resting on bed or chairs will speed up time by 10x so make use of that. Important Notes: More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.