Not real long, I went with my father a few times to fill his 50gal, its just liek filling a truck with a big tank, some full size trucks can be bought with dual tanks that hold 25 gals each so its just like filling one of those.
Grandfather would have a big tanker truck drive down to the farm and fill his, same truck that fills the gas stations IIRC so it has a bigger hose and bigger pump would fill real quick.