It takes me aproximately 2 1/2 hours to do both of my coupons (3G & 4G) on 3/8 plate. Yes the root opening per aws D1.1 is 1/4" and the rod is 1/8" 7018. As far as how long it depends on what method they are going to use to test them. In my experience if it is a visual test the coupon should be 12" long. In a bend, shatter or X-ray the coupon is 6"-8" long depending if you can cut off your start and finish. The only time I have had to certify unlimited (1" plate) it took me eight hours, but I milked it for the pay.
p.s. If the QC is letting you set up your own coupons, I recomend that you knock the flux off 1/8" rod and use that to gap your plate. Then use 3/32" to run your root. Turn the heat up a little because generally the QC will want to see a nice puprple burn on the outside of the backing plate. Also, if you set up your 3G and 4G and alternate between the two, it will give your 3G a chance to cool off preventing dropping your puddle. In my experience it also impresses the inspector.
4 hrs for me on 1" plate, open root 4G. i seem to remember overhead going a little faster than 3G. 3G = 1/8" rod, 4G = 5/32". i usually run the root pass w/ a rod that's 1/32" smaller than what i plan on running for the fill passes when i'm welding w/ a backing plate. for all the open roots i used 1/8" E6010 5p+.