Japanese by Example
Learning through examples in manga

Careless until graduation with 呑気

アオハライド » Volume 13 » Page 18

Aya has mostly given up hope of attracting Shuko’s interest away from their teacher, Mr. Tanaka. Tanaka, having no interest in pursuing a relationship with a student, pushes for Aya to keep trying before the situation changes.

田中(たなか):
呑気(のんき)にしてるとあっという()卒業(そつぎょう)だぞ」
"If you act carelessly, it'll be graduation before you know it."

The に after 呑気(のんき) makes it an adverb.