It's impossible to say with any degree of certainty. As things stand right now with Clijsters and the WSs probably not playing beyond 2012 I could well see Caro be #1 for something like 5 years. The only other current player with results consistently at Caro's level is Zvonareva but she turns 27 this year and I don't see her getting much stronger than she is now while Caro's still rising.
The only player in Caros age group even approaching her level is Azarenka. She's had very good results lately but I'm still not convinced she has the mental and physical stability to maintain this level.
And you never know when a new superstar will burst onto the scene from the junior ranks, but with the physical demands of the game these days we're not likely to see a new Graf or Hingis. The days of players below the age of 18 competing at top 5 level is over, I think.
The most likely thing to trip up Caro's career is injury. Any player can get a serious injury at any time. And she IS playing a rather heavy schedule with a game that involves long matches with lots of running. Caro has an excellent physique and has avoided serious injuries so far but it may catch up with her in a few years unless she cuts her schedule some in the future.
And then of course there's the question of motivation. If Caro wins everything there is to win in the next few years - she well could - she may start to lose interest and not have a very long career or only play the big events like Serena and be overtaken by a new 'Wozniacki'. The Olympics will be an important factor in that regard. If Caro doesn't win in London next year I think she'll maintain her motivation at least until Rio 2016.
She has also mentioned a desire to have children. She may meet a wonderful man and want to start a family. Will it happen at 24? 27? 30? There's no telling.
But, if all things turn out well for Caro: her game continues to develop, no serious injuries, no problems with motivation etc. I see Caro maintaining her #1 ranking almost unbroken for the next 5 years!
True, let's hope the injury bug doesn't catch Caroline. But all athletes can get injured.
We must also remember that falling into a slump is only human. And keeping the top spot while in a slump is not realistic. So while she's likely a contender for the #1 spot for several years, she may have to swap it with Zvonareva or Azarenka or whoever from time to time. Only 7 players have held the #1 rank more than 52 weeks in a row, and only 3 players have held it more than 104 weeks in a row (Graf, Nav, Evert). Steffi has the record with 186 weeks in a row (Aug ´87 - Mar ´91).
Longest streaks, 52+ weeks:
1 Steffi Graf 186
(also 94 & 87)
2 Martina Navratilova 156
3 Chris Evert 113
4 Monica Seles 91
5 Martina Hingis 80
6 Justine Henin 61
7 Serena Williams 57
Getting on that short list would be cool