2-4 hours is definitely the standard length for a game. That said, sometimes games run for 6+ hours just due to luck of the draw. The "Big 4" locations in the base set are the Woods, the Unvisited Isle, the Witch House, and Independence Square. Each of these locations has 10 cards in the Mythos deck which open a gate in their location. Concievably, you could seal off those four locations after the first 8 turns and just keep pulling mythos cards that try to open on those locations that don't do anything. Turn after turn after turn.
When that happens to my gaming group generally we'll all just sit on various locations, skip movements and breeze through encounters until something interesting happens.
But maybe that's not what you're talking about. Maybe you're talking about how you're doing a pretty good job of holding off the slow inexorable approach of Lovecraftian horror and you want to get better at it.
So like I said, sealing off the Big 4 is a good way to get things under control pretty quickly. After those, the Black Cave, Unnamable, and the Graveyard has six cards each. All the others only have two. If you're in a pinch, generally you can close gates in the lower tier locations rather than sealing with without repercussions. Sometimes you'll just have to close a gate to get it off the board. If you find the game is going too slow, going gung-ho on just closing gates is a good way to speed up the doom track.
Getting at least one investigator decked out with weapons is very helpful. People generally want something to protect themselves, but I find that having 1 strong combat character can get more done successfully then 2 or 3 OK characters.
Madness/Injury cards from Dunwich Horror decrease the amount players need to run to the Asylum/Hospital for healing, allowing a larger number of productive turns.
A 5 investigator game is also one of the more difficult configurations for Arkham. Gate limit is one lower than the 4 player game, moster limit on the board is +1, but you'll have twice as many monsters to deal with. 4 player games and 8 player games are the sweet spots mechanically, 4 player games are the sweet spot logistically (Less downtime in between turns keeps players more involved with the game, prevents slowdown, less inter-investigator coordination required, etc.)
Having one player that knows the rules decently well is pretty important. You'll be tempted to pull out the rulebook and try to find specific answers to scenarios, but it's generally a bad idea. Make a ruling, remember to look it up after the game is done, and keep playing. Stopping the flow of the game to look up rules can really extend game time and kill enthusiasm.
Try not to take breaks during the game! Maintain focus! 3-4 hours is pretty long time to play a board game for some people and if they're fiddling with a smartphone, playing on a DS, watching a TV that's on in the background, taking a break to get food, talking with another group of people, knitting, etc. it takes them time to catch up with what happened in the game when they weren't paying attention, people have to wait for them, etc. I've had 8 player games done in 2 hours just because everyone was really paying attention to what was going on in the game.
That all said, even experience groups can find themselves in a slogfest (or a shogfest har har) and you may find that taking a collective break and coming back does wonders for everyone's morale. Group morale is a pretty big part of the game interestingly enough and when it starts going down game speed starts going with it. Have a decent grip on the rule basics, maintain group cohesion, and have fun; soon enough most of your games will land in the 2-4 hour window!