For me I had a GF and then got back with her 5 years later. I assume people out there have much longer times between 2 relationships with the same person.
My grandmother’s brother married an 18 year old girl when he was 46. He died at age 67. His widow then married her high-school sweetheart. So there was at least a 21 years gap.
Though we were 600 miles apart, I saw my now wife fairly frequently from Jan '72 - Jan '73. Then we broke up. I visited her again in September '73, the next time in July '76, and then July '77 when we got engaged. So I’d say the gap is Jan '73 to July 77. We never could stay apart successfully.
What broke you guys up in the first place and brought you back together?
I hooked up with my wife in college in 2006 and then didn’t see her again until the summer of 2011 and then a couple of years later we got married. So a little over 5 years of course we weren’t SOs in college just friends who hooked up at a party so I’m not sure it really counts.
Broke up in 1972, reunited in 1979, broke up again after maybe three dates.
We got married after a 33 year break.
Four years. Dated 92-94, broke up, got back together in 98, dated most of that year and split again. She’s still married today to the guy she left me for.
This doesn’t entirely meet the OP’s question, since we weren’t actually dating the first time, but it’s still a fun story.
I met the woman I eventually married in early '84. I was 18, she was 20; she had gone to high school with a friend of mine (all of us were in college at that point), and she had come into town to visit my friend for a weekend. She and I and we were both in relationships at that point; I thought she was nice, and funny, but didn’t really give her further thought.
I didn’t meet her again until the summer of '89 (I was then out of school, and sharing an apartment with our mutual friend). Even then, it was just a one-off re-meeting, as she was only in town for a short time, but we were both single, and hit it off. Six months later, she moved to the town where I was living, and we started dating soon after.
Not really a relationship, but I did have one-time sex with a guy I had dated twelve years earlier. Mostly it was confirmation that you can’t really go back.
14 years.
Dated in 10th grade. After graduating high school in 1995 we didn’t see or speak to each other at all. She moved back into the area in 2009 (I never left); we were living in the same suburb about a half mile apart and ran into each other. We got back together after catching up, moved in together after 4 months, bought a house together 3 years after that, got married 1 year after that.
Similar story: was involved with a woman in 1981, we broke up and didn’t see each other for years. Ran into her again in 1987 and had a brief fling but not enough commonality to sustain anything more.
My aunt has been married eight times.
Husbands no. three and five was the same man but I don’t know how many years were in between.
She went off to college in 82 and we broke up. 5 years later in 87 we got back together but that only lasted 3 months. I see her now and then after not seeing her for 20 years. She was married for a short time and then got married again but now she is separated the last I heard.
I dated a woman 20-ish years ago. We broke up because she wanted marriage and I did not. We ran into each other recently and she said she missed what we had and asked if I’d want to mess around. I explained that I was in a committed relationship and she said OK.
Afterwards I found her Facebook page and it turns out she is married.
About 10 years ago, my maternal grandmother, who was a widow, looked up her old high school sweetheart. I’m not sure if it was via the Internet or a phone call. Turns out, he was a widower. Well, they hit it off and one thing led to another and a year later they were married. He died about 4 years ago, but the marriage was a good one and both were very happy to have re-found each other, even if it only lasted 5-6 years.
So, doing the math…They date in high school for a couple of years and then go their separate ways. Lets say 17 years old. Re-connected at age 85. That’s a ~68-year gap!
My aunt dated two guys in high school. She eventually married high school boyfriend A. Twenty years or so later, they divorced. Not long after she married high school boyfriend B. They’ve been together twenty plus years.
I talked to a boy a bunch on a local BBS when we were 13 or 14. We finally met and spent a day together at a local amusement park. We never really spoke after that.
Some years later, when I was about 27 I think, we connected again and dated for several years.
I was with a young lady for a couple of years right after high school. We broke up in 1991. In 2011, I found out that she lived in the same city as me (in a different state from where we grew up), so I looked her up. We’ve been together ever since. So, 20 years.
My uncle had a similar situation. A few years after his wife died, he looked up his high-school sweetheart, and they wound up dating for a while. The relationship didn’t work out in the long term, though. And this was when he was in his early 70s, so his gap wasn’t quite so long - “only” 55 years or so. 
Met in 1971 in college, dated steadily, and broke up in 1975. Didn’t know anything further about each other until 2007, when we started dating again. Got married in 2013. Soooo…32 years.