A number of years ago I was hanging out on the (New) Jersey shore and came across a tangled mess of seaweed, fishing line and a section of rope about 2 feet long.
I set about untangling this mess attempting to recover anything I could use if I happened to be washed up on a remote beach. That's the type of really odd stuff I do at the beach when I'm not body surfing. I ended up with an assortment of hooks, snap swivels, sinkers and mono-filament line, enough for a short hobo fishing rig. The line was on the short side and a two foot section of rope was not too helpful.
I took the rope apart and made about 30 feet of light 2 ply cordage out of it. That gave me enough to tie the fishing line to for some serious reach.
In September I'm planning to teach the cordage portion of our bushcraft class and may use the rope as an exercise. Anyway yesterday I figured I'd give it a run and time it and also measure the length of cord I could make from a 20 inch length of 1/2 inch hemp rope.
It took me under an hour and I got 18 feet of paracord sized 2-ply. It is good practice if you care to try it.
I didn't take any process photos but basically you unravel the rope to its smallest components, these individual bundles of fiber are then divided in half and retwisted into 2 ply, spicing them together as you go. You should wind up with loose fibers that can be added back in to keep the sides equal if one gets thin. I ended up with only a small tinder nest of wasted fiber.