Well thats a varied answer I'm afraid! I would say the longest casual 300 point game would take about 1.5 hrs including set-up. Lesser points would be quicker typically, but I have played 100 point games that took over an hour so it really depends. I'm playing a 3000 point game, still on turn 1 about 4.5 hours so far!!

@OP
Its a great question, and I'm not sure if they keep the tournament data to hand for that kinda thing - maybe Hutts could chime in here with some stats?

>200pts: 30mins (assuming 'league level' skills familiarity) to 1hr,
250pts+: 1HR+

As EmperorDragon & darthwhovian remarked, it can depend on those shatterpoints - a critical mistake can lead to the game ending quite quickly. Such as not moving your major close combat piece into cover ahead of time. So, stuff that activates a character/stuns them, Big Disintegrate (horrible in RPG/scenario gameplay, especially if you've only got one life and you are disintegrated by a Gotal), reserves, repair, heal, recon can end a game or prolong it... Door Gimmick without a Satchel Charge balance can be excruciating .

Flipside of the coin,
themed armies and Nom Bomb squads tend to, from my experience, be a much shorter battle duration.
Tusken Raiders or Ewoks, any swarm-based theme army is going to be short work for a Meta-competitive squad in the hands of a Min-Maxer power gamer.

Jawas against droids though, that was Nasty (ion gun and twin/double attacking Jawas).

This is all assuming there aren't any objectives for the Player Characters to achieve - if they must do 'capture the flag' and Monopoly around the board before a victory condition, you can easily pad some extra time in. that might even be the Game Masters Ace up the sleeve as a hidden victory condition, should things be running too ahead of time. Conversely, if you've got any map tiles/terrain handy, you can remove objectives on the fly if things are running over time - say there's 3 objectives they've gotta achieve by round 6, but you're 40 mins in and only up to round 3/4... you could blow up one of the objectives (furthest from the players) and say it was from NPCs...

Happy scenario playing, and it'd be great if you happened to turn it into a game report .

Scenario rules and house rules always have an influence on things, and so will each player's play speed. Usually for me, a 200 point game will take about fifty minutes to an hour and fifteen. A rough estimate I've noticed is that adding 50 points to the squad total tends to add roughly thirty minutes to the game, assuming only two players. This isn't a hard and fast rule, though, as game times can vary wildly. (For example, I've had 500 point games take only two hours.)