If you work in consulting, anytime between 3-5 years is optimal. If you work in corporate, it really depends on your experience and how well you can build a good case for your readiness for school. The average years of experience for top schools are around 5 years for all students (3-4 years for consulting or banking folks and 6-7 years for corporate folks).
I don’t think 2 years of experience anywhere is enough for anyone to truly get the most out of business school. Many people go because some consulting firms expects you to apply. Even if you pass the application process because you work for a hot shot firm and you built a good story for your apps, you may not really know why you are going to school and what you want to learn. Business school presents vast opportunities to network, experience, and explore new areas. If you are not sure why you are going even if you seem sure on your application, then I would advise to wait a year or two.
There is no rush. A sense of purpose and a level of maturity really help you focus on getting the most out of school.
There is however, such a thing as waiting too long. Once you make manager in consulting, business school is a waste of time and money unless you are burnt out and your firm is paying for your school. Then by all means, enjoy. Once you have more than 10 years of experience anywhere, consider executive MBA instead or weigh seriously the time/cost of stopping your career for an MBA. The only good reason to do that is for a change in career.
At the end of the day, it depends on your specific situation. These are just some general guidelines. If you want me to comment on when you should go, post me a comment and I will respond.