I was at the Expo on May 19, day tripping from Florence, which gave me about 8 hours at the site. I was able to see 5 major pavilions (Italy, Germany, USA, UK and China) and several smaller ones, but it wasn't enough time to see everything I wanted to see. The Expo is less busy in the morning and after 17:30, before the school groups arrive and after they depart. The Expo is big but not huge, so I think you could see most of what you want to see in 2 full days (opening time to closing time). Not sure when you plan to go, but if it's summer and if summer is expected to be the busiest time, then maybe a third day could make sense.
I thought the Expo was interesting enough that I plan to go back for another day in September. I'm especially looking forward to seeing the France and Japan pavilions. There are also some great food options: Eataly offers food from about 20 different regions of Italy (a bit pricy) and several countries offer traditional dishes. The best food prices are at smaller pavilions off the main concourse, for example Algeria's lunch buffet was priced at only 10 euro. Czech Republic and Belgium offer their famous beers and have relaxing areas where you can sit and rest while enjoying them. Everything is in Italian, English and often the pavilion's native language, so English-only isn't a problem. The train and metro station has exits directly to the main entrance, so getting there from central Milan or even other cities such as Florence is very easy.