If I wanted a leaderboard for a demo version of a game, how 'substantial' should it be? As in, how long of a speedrun/how much content? If you get what I mean; I understand the question is quite opinion based arbitrary, but would a run of about 2 minutes be accepted? I assume not, but 5? 10? etc; I know that Deltarune got accepted even though it's technically a demo.

I guess I'm just unclear on the site's stance on "demo's" in general.

We tend to reject demos in game requests. It's not about the length of a run, but more about the uncertainty about the final state of the game. History has shown that demo boards can be hard to maintain and/or transform into the final game's board. So many things can change during development and so many boards in the past have ended up in unsightly states with different versions as categories or weird mixes of incomparable runs on one leaderboard, etc. To avoid that, it's best to wait for the actual release version before requesting the game. Saves everyone a lot of trouble. Nonetheless, we still look at every submission individually and check the update history and release roadmap if there is one to make a proper judgment, so in certain cases we'll allow games that are very close to release or are unlikely to change in content/balancing that's relevant for the speedrun.