the longest verified run is just under 340 hours

If they are alredy accepting slowruns with the whole purpose of being the longest possible, having a limit time for the slowrun over the 340 hours that is alredy a thing would be a nonsense Now, no one ensures you it will be accepted if you do a slowrun: mods of the games are always on the right of banning those "speedruns" that cleary do not attempt to actualy be fast (more if instead they do the exact opposit, with characteristics like waiting without doing nothing for 8 hours straight or that kind of stuff) But there are alredy a few longruns of hundreds of hours in the leaderboard, so apparently there should be no problem in this lb to submit runs as long as you can

good luck on the run btw

Speedrun.com site rules (https://www.speedrun.com/knowledgebase/moderation-rules) state that "a minimum effort requirement is reasonable" and "runs that are clearly not making an effort to beat the game's category quickly may be rejected." While there is no hard limit on the length of a Minecraft speedrun, verifiers reserve the right to reject slowruns. I would not waste your time or theirs on such pursuits.