The only differences in a multiplayer campaign versus one that is solo are:
1. There are more human players, obviously, so they can interact and work together, come to each other's aid, etc. It's important to use the flare key (F), and voice chat or text chat -- voice chat via an external program like Skype or Teamspeak is preferred.
2. You can give ships between players by selecting ships and clicking the little Give button at the bottom of the screen. You can only give ships that the other player has also unlocked, and that would not put them over their ship cap for that type.
3. There are twice as many simultaneous AI waves into your systems as there are human players. So when you play solo, you only have a wave or two at a time. With four players, you have four or eight coming in at any given time.
4. The AI planets have more ships, and get larger reinforcements.
5. The ship caps for human players are around 10 smaller per extra human player beyond the first. So in a 1 player game you can build 170 mark i fighters, in a four player game you can build 140. Your combined forces are much greater, though, of course.
6. Your team succeeds or fails as a group. No one is "out" early. So as long as one player has a home command station surviving, your team can still win. It's the same for the two AIs in all games, actually.
7. If a player hits the resource storage cap for metal or crystal (300k in 1.013, 600k in 1.014 prereleases), then the excess resources flow to allies instead of just being wasted.
So, as you can imagine with those few differences, the length of games is pretty much the same in either case. This game was designed around the concept of co-op from the ground up, so the single player game is just "co-op by yourself," which is also fun but therefore not as different from multiplayer in terms of mechanics as some other RTS games are.
Being able to finish a 7-hour game in one sitting is of course ludicrous for most people -- myself included -- so that's where the multiplayer save comes in. Like playing a game of CivIV in multiplayer, you can play off and on with a group over a period of days, weeks, or even months. At any given point I tend to be in the middle of around 7 different AI War campaigns: one with my wife, one by myself, and then around 5 others that are the various permutations of my alpha tester group of four players. So if someone is missing on a given week, or several someones, then the remaining ones of us just pick up where we left off with another campaign.
Sometimes that means a month or even several months has gone by since one of those side campaigns was last played, but with the Notes feature in the galaxy map we can always remind ourselves of what we were doing and what was going on. It can actually be pretty cool to come back to a cold campaign in a later version of the game, and see how the new DLC and options has impacted the options available and strategy for the older campaign.
Anyway, the length of time a game takes will vary greatly with your playstyle and how big of a map you choose, as well. Smaller maps are faster, but I feel like the strategy is really reduced below 40 planets or so. It's a very different game with 10 planets, for instance. I tend to play only at 80 planets, but those take more like 12-16 hours for me. The difficulty of the AI level you choose will also impact playtime -- if the AI is overmatching you a bit or is right at the tip of what you can handle, then the game will be way longer usually, since there is more back and forth. If you play where you are comfortably challenged, but not challenged to the edge of your skills, then the game will last a more average amount of time. Or if you want to crank the difficulty down and just steamroll the AI and play around with the mechanics, then those go very fast. There's such a wide range of possible difficulties in this game that you can find levels that your mom can play (or your senator), and there are levels that no one has actually beaten yet that I am aware of (a few expert players came close to beating a pair of 10s in past versions of the game, and some players have done okay against 10s when giving themselves a big handicap bonus, but that's it so far).
The new version 1.014, which is still in beta and so has some bugs and kinks at this point, also really speeds up the game because the economy of the players is so much stronger that it lets them do more stuff more quickly. But even that will only likely shave off a few hours from a big game, I think it would reduce a 13 hour game to maybe 10 or 11 hours, but I don't have a lot of data on that in practice yet.
Playing on the Fast & Dangerous combat style instead of the Normal combat style also makes a huge difference in game length. I play only on F&D, as do most other traditional-RTS fans, but the people who are more fans of TBS games, and new players to the game, tend to prefer the slower pace of Normal. F&D is around 30% to 40% faster in terms of total game time on average, I'd say. Somewhere around there.
EDIT: And yes, this is in the wiki.
