You can spend hours and hours.
It can be a little difficult for newcomers and it takes some patience to learn and master. But when you do, you can do whatever you want: Build you own home town, make a dull farm town, build a bussing metropolis. The choice is yours.
You set your own goals and work on your city to reach them, and then set new goals. The downside is, that you never really finish the game.
The different Sim City communities on the net, gives you the possibility to tweak and expand the game for years to come. There is tons of content to download for free, and you can create your own content to share with others. If you get stuck or need adwise, there is always someone to help in the forums.
The game is now more than 4 years old, but the graphics still seems nice and crisp. And you really need a modern computer to run the largest cities with all the details on max.