since we weren't really given an exact timeline, we can only give inferences. The most solid proof that's it's been a year already is through Sangwoo's dialogue.
Why do you think it's been a year?
anniversary: /ˌanəˈvərs(ə)rē/ :: the date on which an event took place in a previous year.
they were probably referring to sangwoo's dialogue in the latest chapter, stating that it was his and bum's "anniversary"
DEM SON. "Apply water to the burned area."
Has Bum been with him for a Year now? If so that has been a long time and he really dose have Stalkholms now for shur.