I went last year (2018), it was awesome. Here are some suggests from a 'first timer' there.
I would say one day for the official show if you get there at the opening when it is not too crowded, and one day for the camping area, which is just as interesting as the show.
I would recommend Friday, as on Thursday a lot of vendors have not arrived or are still setting up.
I had a car, but a train is a great option. Trying to drive to the show is a nightmare++. All visitor have to take a shuttle from the parking lot, and the vehicle line to get into that can stretch for 6+ kilometers! I highly recommend staying at Campus Bad Kissingen, Schurzstr. 2-6, 97688 Bad Kissingen, Germany +49 971 722611. These are school dormitories that they rent out. You can get a private room with a private bath. It is located less than 1/4 kilometer from the parking area where you catch the shuttle to the show. A perfect location, very clean and quiet and they have a wonderful buffet breakfast that is included. And a very reasonable price. I use Booking.com and it was 54€ per night for a private room and private bath. A very good Greek restaurant (El Greco) is less than 100 meters away.
By the way. Contrary to the normal Germany well thought out, planned and organized way of doing things, the shuttle loading area is a free-for-all (think Bangladesh train station). No lines, no guidance, not organized bus arrival, so be prepared to be assertive to get on a bus, if you get there at the opening.
The camping area is amazing. I was there on Friday and Saturday. It really filled up on Saturday. I enjoyed it as much as the show. I had a hard time finding its location ahead of time on-line. And even when I was there, I could only find it by following Expo Vehicles, until I got close enough to find temporary signs. It is at the water treatment plant. The turn-in road is across the street (west side) from Casino 13, Würzburger Str. 27, 97688 Bad Kissingen, Germany The campground is 1.5 km from the train station, and the parking lot for the shuttle to the show is about 5km from the station.
BTW if you are flying into Frankfurt, I found it much cheaper and faster to rent a car (about 1/2 the price and time of a train). Just be aware that rush hour Frankfurt traffic is 7 times worst that Los Angeles traffic so avoid the rush hour if you want your sanity.
Also VERY IMPORTANT, Thursday, 20 June is Corpus Christi (Fronleichnam) day, and 99.5% of everything is CLOSE. No restaurants, no grocery stores, no shops. Gas stations are the only thing open on that day.
We've been a number of times and the campground gets very busy, we try to get there on the wednesday and already the campground is filling up, It does not help that some of the Locals reserve large spaces by roping off favourite areas. this is a PITA and needs to be addressed by the organisers at some point.
If you are camping get there at the latest friday lunchtime.
The above comments about the shuttle queue are spot on, take a later bus - they run every 20mins or so or sharpen your elbows.
I've gone twice now on Sunday the family day. Getting to the parking area seems like a pain and bus ride is slow but it might be because I'm so anxious to get in! Highly recommend bringing a backpack with snacks and water. It gets hot and not much shade.