1. I was there for about 1.5 hours. I got there around 9am. My tour was super short because HIPAA regulations don't allow just anyone to go where patients are at (confidentiality issues, perhaps). So I just hanged out with the med students and talked to the physicians, which was really great. They were AWESOME! BUT, if you are flying there and staying at a hotel (like I did), I don't think it's worth staying for the clinic visit. If you've volunteer at health fairs/clinics, you already have an idea of what kind of work they'd do at these clinics. That's just IMO, though!
2. When you get there, there'll be some snacks/juice/coffee, you'll chat with 2 persons from the admissions committee...very casual and they'll answer any questions about the MMIs. You'll then have an info session. Then you'll either go to your MMIs first or be taken on a tour and to meet the dean (he's super nice!). After that, a delicious lunch where you chat with students. Then you'll either go to your MMIs first or be taken on a tour and to meet the dean (he's super nice!)...whichever you haven't done yet. Then a wrap up session.
3. Like the other poster said, just relax, be yourself, and don't overthink. They just want to see how you handle stressful situations. Oh, and if you want to stay with a host, do it. I've done it at other schools and it's really helpful as you can ask about anything. My host at UC Davis got back to me really late so I had to stay a hotel, but the UCD students are very nice. Best wishes!