I'm currently working as a Consultant (started in August 2012) in London. I plan to move back to the States through intra-firm transfer. The career promotion track in the UK and especially London is not MBA focused as the United States. However since I am planning to move back when would be the best time (in terms of number of years of professional experiences) to apply for a MBA back in the US.
There are two answers your your questions... however, neither of them are as simple and providing a number. The first, and most relevant, is however much work experience it requires for you to obtain a significant story to sell yourself to the admission committee. The second is different for every school. Go to the websites of the programs you are interested in and you will find general information on their classes including the number of years of work experience. Good luck!
kudos are appreciated