Manager Mode is part of the Career mode in FIFA 12.
Manager mode can in theory be played forever because of the fact that there are youth players always being produced.
To have a successful manager mode you need to:
Keep the board happy by completing objectives.
Mostly these objectives include winning titles or progress in cup compitions.
Here are a few you may encounter:
Win the League
Win the Cup
Reach the Semi Finals of the Champions Cup
Finish in the top half of the league
These are a few you may encounter or ones similar to this.
Another important part of manager mode is your Youth Development squad.
Doing this can make you a successful manager because there is some real talent out there.
See the Youth Development article for more details.
Transfers also play a big part of Manager Mode.
Appart from Youth Development there would be no other way to improve your squad.
You can loan or buy players.