We did the same on our ED a month and a half ago. From what I remember, the tour ended in an hour, and we hit the road right after.
I would certainly not count on the tour finishing in an hour. As turpiwa stated above, it's about two hours. Same in May as it was five years ago. If you count on it being an hour and it runs the full two hours, you can't just leave the tour and go back to the Welt, you're there until the tour goes back to the Welt.
Regarding gas, when we did Euro Delivery five years ago for my wife's 335is Coupe, we had only about a quarter tank of gas. We had a tour at the Dingolfing plant and we couldn't find the gas station on the piece of paper we were give at the Welt. We looked at the distance on the Nav, looked at range on the dash, and decided we had enough gas to make it to Dingolfing.
Last May, for my M4 Euro Delivery, I had a full tank as did everyone else in our group delivery, a total of 13 M cars. But don't assume that you're going to have a full tank and you won't have to stop for gas, assume that you will have to stop for gas and plan accordingly. Better to be early than late. That goes for every other part of your Euro Delivery. Don't try planning things down to the wire because you can never plan on traffic or weather.