I'd agree with watchdog for the most part, other than to say that if your adventure or scenario runs a bit long, try first to cut it down to fit into the 4-hour slot you were originally targeting. But if you can't find some element to remove to speed things up, defintiely give yourself that buffer, yeah. You want your convention game to end up a high note - better to wrap up 20 minutes or so early and chat about how fun it was than to rush to not even get to the final encounter and climax.
With regards to miniatures games at 6 hours or more, there were only 68 in 2015, and this is where we get into what I was talking about erratic data. There are few enough events that hugely successful (or not) event can sway things one way or another.
Looking at general trends, though, the games overall averaged 18 out of 36 scheduled players (or 26 average max, if we remove ongoing drop-in/out events, like Iron Arena). Average individual attendance is around 60%.
Events that were both 6+ hours and better than that average 60% attendance seem heavily weighted by Privateer & Fantasy Flight premier tournaments, as well as those from some other groups (like SWM Gamers, Able Kompanie, etc.).
Removing tournaments leaves around 10 events. Aside from a big Battletech game, the remaining games average 12 players (both scheduled and actual), and are a mix of new and old: Battletech, Mordheim, X-Wing, Dust Tactis, Mordheim. Looking at the titles & descriptions (as someone who is passingly familiar with minis but certainly not deeply immersed in them), all but the Mordheim events seem to be special. Either it's a big, chaotic event or it's got a very cool scenario beyond "let's just play a game of minis."
Looking at the lowest performing events (and removing those for which we have no ticket data at all), it's almost entirely tournaments, mostly from specific groups who know who they are :). Aside from that, the most common thread is a bland title or description. Most (not all) of the events basically boil down to "Come play this game. Here's the point value to build your army." Broadly, they have 7 out of 18 scheduled players.
So I'd say that supports the broad assumptions I drew from general data previously. If you're going to 6+ hours, just make sure you consider your evnet carefully. Odds are, just having a simply tournament may not be enough and you'll want to trim down, unless you have a really cool scenario or idea.
Gen Con LLC