Here is a sticky that answers your question
MD/MBA Program List
And here is a good website for additional reference or information
MD/MBA.org
Quick short answers to you question - It depends on the school. It seems like the trend is 3 years of medical school then business then combined for both.
As for when you apply, that also depends. I found the sticky very helpful in answering that question...and the link to the website isn't bad too. Also a nice google search that says XX Medschool and MD/MBA or XX Business School and MD/MBA will start you on the right path.