What's the average game duration? I've limited time to play, and would like to know if it would be feasible to go online.

Depends what you are playing, either single player or mutliplayer. of course in single player you can go for more than a dozen hours just for one game, but for multiplayer, 1-3 hours (no clue cause I rarely play multiplayer)

Is there an active community I could join (newb friendly then), preferably composed by european players, so I could enjoy compatible timezones and better connections?

Not really since this is a small game.

If so, is it using any tools to facilitate game scheduling, like Steam?

What do you mean scheduling?

Why not just post on here instead.

1. Game times are typically around 1-2 hours, rarely longer.  The reason is that (unlike singleplayer with an AI) there is usually a gentleman's surrender by the losing team once they know it's only a matter of time before they're defeated.
2. There are good mannered players... and bad mannered players.  If you can take those things in stride, you should be fine.  Above all, just keep an open mind and take the suggestions of your allies.  If you take some time to browse the strategy forums you should get your bearings fairly quickly.
   
3. Presently?  No.  I believe that feature is currently in development.

Most games, at least the ones I've played, don't last more than 2 hours and those that do tend to be epic games which you won't mind playing for a third hour anyway.  As a general rule of thumb, you should count on having 2.25 hours available before you join a game unless it's a 1v1 or 2v2.

Your best bet for finding new players would probably be to look for games in Regular Sins.  However, most of the action is for Entrenchment which is where you'll find the experienced players and the 4v4 and 5v5 games.  Note that you can also play humans v. AI games (also known as "comp stomps"), which might be a good way to chat with other people and ask advice and to watch what other players are doing.