It depends on the school. We have a lot of research integrated into our program. Last summer before starting our clinical coursework, all of us did research as a part of the curriculum. Several of my classmates have continued doing research during the year, and a couple of them even presented at an AMSA meeting last month as M1s on work they've been doing since they've been in med school. We will all be doing research again this coming summer, and then we will all be doing a year of research too. If you're interested in doing a lot of research while you're in school, you might want to see if your school offers an optional MD/MS program. They usually don't charge you for the fifth year (or they don't charge very much if they do charge anything) and you can apply for funding too.
Eh, depends. I'm looking at doing something more than can be reasonably accomplished in one summer, so I'd use some elective time to wrap up my work. If you've done the leg work setting it up beforehand, I'd imagine that you could analyze and write up a few case reviews and/or a retrospective during a research elective. I've heard about people doing something like this when they realize that they like a competitive field last-minute and want to bulk up the CV.
