If you're referring to the MMI, it's 1 to 1 with the interviewer/assesor. It takes around an hour, sometimes more, sometimes less. Mine was about 40 minutes but effectively it takes as long as it takes for them to get the info they want from you.
Beforehand you'll be given a sheet of paper with 6 questions on it, asking for different examples of a time you did something. The wording can change but effectively they're always the same sorts of examples. You can find them by searching the forum at the top. You'll be asked to give a very brief answer in writing of the example you have. They'll then take that away to formulate some follow up questions.
During the interview, you'll be asked those same 6 questions, but here's where you give as much detail about them as you can. Remember to follow the STAR format as it keeps you focused on the question and the info you want to convey to the assesor.
Try to relax and give as much info as possible, and remember it's about what YOU personally did, if your example is about when you were a team, keep the focus on what your part was.
You'll be asked follow-up questions like how did it make you feel after doing the good job you did and things like that.
Good luck.
