Approximately 1 year later.
The Rise of Skywalker Visual Dictionary contains a timeline of Finn's life.
The Battle of Crait, seen at the end of The Last Jedi, takes place on 0 ASI (After Starkiller Incident). The mission to the Sinta Glacier Colony, seen at the start of Rise of Skywalker takes place in 1 ASI. Since the smallest unit used in these timelines seems to be .5 of a year, the longest the gap could be is 17 months.
