A straightforward 1v1 game might take up to 2 hours at most, unless the players were either very well matched or extremely cautious.  Beyond that it gets more complex, the games can be either shorter or longer depending on how they go.  Any 'FFA' game might take a very long time, same with any multi-star map.  But if you stay with the smaller maps- 6 players or less- two hours would be a longer-than-average game. 

That's based on 'fast' settings, as used for most multiplayer games, and also all human players- team play against the computer is probably longer, but I haven't tried it.  I've seen resumed 'comp stomp' games from saves though, this very rarely happens with all-human games.

If you're new to multiplayer you would have to be very good to still have a chance 1v1 against an experienced player after about an hour and a quarter though. 

If you want to finish an sp game, try a smaller game...?

Most people don't play multistar games in multiplayer, they just take too long.  When they do, it's almost always a 5v5, so you don't need to manage everything yourself.  Games seldom run over 2 hours, though I've had multiplayer games run up to 4 hours before.  They can be a lot of fun and are quite intense.

There are essentially three different varieties of military victory:

The first is a victory of attrition.  One side is either killing units faster than the enemy or has a stronger economy to replace them more easily (or a combination of the two).  Slowly, the winning side is just going to erode away the losing side.  Unless the losing side can do something to turn things around, this will eventually lead to a catastrophic defeat some time down the road.

The second type is a catastrophic defeat.  Sometimes, one side is just wiped out by a specific battle and the vast majority of their fighting force was destroyed.  While this is sometimes survivable in team games if an ally can come to your aid, it usually decides the game.  Even if you could replace your forces, how do you expect to win against an enemy army that already wiped out that army once and has only gotten stronger since?

The final type of victory is a tactical victory.  One side has a better defensive position than the other, even though the two sides might still be evenly matched in a literal sense.  In a strategic sense one side now has a massive liability that they simply don't have the resources to account for.  This usually leads to a war of attrition where one side has an easy time defending and the other side has a hard time defending.

Often times you can win one of these types of victories relatively early.  The AI doesn't recognize that it's already defeated and is fighting a futile war.  It will often fight a futile war for hours on end after its effective defeat.  This is what makes singleplayer games so long.  A multiplayer human isn't going to be that way.  He's going to make a decisive push and once he feels his victory is assured he will ask you to surrender.  If you have some surprise up your sleeve for him, sometimes you can turn things around.  If you don't, it's considered horribly bad-mannered to hold out on someone who has effectively won the game.

In team-games, usually the team will decide to surrender if it's losing one of its front-lines but isn't winning somewhere else.  It's the same deal; players know when they're beaten, and don't need to fight for hours on end when they've already lost.

Your average online multiplayer game only lasts 1 hour, 30 minutes.

How could that be?  Because the games you are playing are probably you v. 9 AI on a huge multistar map where you have to knock out all 9 AI.  In contrast, a multiplayer game is more likely to be 5 humans v. 5 humans so no one player has to win all of the battles and take down all of the planets--if you're on the winning team you have 4 allies to help you and only 5 opponents (and not 9) to kill, so you really just have to kill and bomb out one single opponent yourself.  Some games only last an hour and epic ones can go on for 3 hours.  Also, normally once the outcome of a game has been decided, the players on the losing team will call it "gg" (good games) and quit or surrender so that the winning team doesn't have to go through the boring motions of bombing out every single enemy planet.

As a general rule of thumb, you should budget 2 hours for a game.  I think 90% of them are decided and over at the 2 hour mark.

When you have a good chunk of available free time, log onto ICO and check out the online multiplayer action.  You will discover that human opponents are much more challenging than the computer AI and that the games are more suspenseful, more intense, and more fulfilling (at least when you win).  You will also get to enjoy team-play-based aspects of the game that are not present in single player.  If the thought of playing against human opponents is intimidating, you can also play humans v. AI comp stomps, which might not be a bad way to ease into online multiplayer.  Do note that regardless of how good you are against the AI, playing Sins against human opponents is a very different game in terms of the strategy and it is much more challenging and more difficult and that you will probably get creamed for your first 20 games or so until you learn how to hold your own.  (Even heavily experienced pro players get beaten down by other pro players.)  However, if you stick with it and persevere and try to learn the game and its strategy you can become a good player over time.