Just found these two former jorneymen of 90s met in 2024. Their 1st fight has occured in late 1999. The time gap was incredible 24 years and 3 months. Were there any longer? https://boxrec.com/en/event/894372/3145815 I know that Larry Holmes and Mike Weaver met in the rematch that happened 21 years and 5 months after their 1st meeting.
There were more than 16 years between Nelson/ Fenech 2 and 3. 17 years between RJJ vs Hopkins 1 and 2. 24 years is going to take some beating.
Since the obvious ones are already taken i will add Mike Tyson vs Henry Tillman. If I'm not mistaken Mike lost twice to him in the armatures then knocked him out in the pros. Its not a lot but it is still 6 years. Do recent fighters count? Haney and Garcia fought in the armatures 6 times around 10 years ago only for Garcia to knock Haney on his but over and over most recently.