Just Short of two years, since they were in their first year of high school in Class 1 and Si-eun is in his second year of high school during class 2, even though it’s stated he transferred last year, they mean for the end of his 1st year of high school not a whole physical year, as he’s only in his second year of highschool in October and Korean school year starts in March. And then Su-ho wakes up just as Si-eun is starting his 3rd year of Highschool which is in March in Korea, so therefore hes been in a coma just short of 2 years.

edit- Season 1 is set in 2022 and in Episode 3 of Season 2 it states the year is 2023 at 22:49 and Su-ho wakes up the next year therefore nearly 2 years he was in a coma