is it possible to ride 8 miles on a 20" uni and if so how long would It take??
I have done close to that with some walking and it took about 6hours. I went really slow…
Yeah, definitely possible. I rode 5-6 miles in an hour going mostly at a casual pace with a few spurts of “let’s see how fast I can go on a 20 inch” pedaling.
I really want a 36er. 
I don’t ride my 20 much anymore especially for further distances but if I rode while going for a “walk” with other people I would be riding a 20. 8 miles is definitely not a problem especially if you have stuff to distract you and are not in a hurry to go anywhere.
It used to take me forty minutes on a 20 to commute 6km when I was still new to the sport. Now it takes me 12 minutes (16 in winter) to commute 4kms on my 36 
Possible? Sure. Comfortable? No.
It would help to use shorter cranks.
I had a photo somewhere of a Japanese rider at UNICON 2002 who rode his 29er in the 10K with I think 70mm cranks, but I can’t locate the photo at the moment. Foss or Harper could possibly clarify the story for me.
We’ve had people do 19 miles at the LBI Unithon on 20" unicycles a few times. It’s a very very flat route. If I recall correctly a couple of people managed it in under 4 hours for sure perhaps even as little as 3 1/2. So, yes, 8 miles is certainly possible and doable in a couple of hours.
Multiply the number of wheel revolutions per minute by 0.0595 to find your speed in miles per hour.