I have been looking for a while to find the world record for the longest Dungeons and Dragons marathon because me and my friends had a plan to try and attempt to beat it and give the money from the stream (if we get any) to my school's Gaming and Anime Club.
The record seems to be 209 hours (= 8 days, 17 hours).
According to this Reddit post, Reddit user JustOneAmongMany asked Guinness about an alleged 1985/6 record. Guinness responded on September 4, 2018 with this message:
Thank you for contacting Guinness World Records.
A search of our database shows two Dungeons & Dragons endurance records from that general time period:
From June 30th through June [sic] 3rd, 1986, a team of eight players (six finished) played for 66 hours in Ilford, Essex.
A team in Ogden, Utah then got the record for a 1983 game that lasted 209 hours.
If you want more information, you should contact Guinness here.
I was part of the 1985/86 group, at that time there was no official record. There was a 5 minute break every hour but you could stack them for a longer break. We did several 24 hour sessions beforehand to train. We were dosed up on coffee and coca cola. We practiced in Ilford but the record was set just off Trafalgar Square in some sort of basement of a building there. Possibly the disused underground station room but I can't recall detail. There were oringinally 12 of us, 8 started and two dropped out, but the others not playing helped with buying food and drinks and giving support. Think we started at around 3rd level and went up to about 9th, having to bring in characters when necessary.
This thread suggests that the record was/is 100 hours
in Dragon issue # 80, pg 54 it states that four members of the Broward Game Players Club in Hollywood FL played D&D for 100 hours straight.
However that was back in 1983, so it might have been broken since then.