Yarn harvesting, sweater recycling, whatever you call it, it's great fun (at least for this nerd) and super thrifty. I have a thrift shop near my home that sells sweaters for $0.25 each on Thursdays (1/2 off day) and I can usually get 5 or 6 skeins worth of yarn from one sweater. I never measure the yardage, but you can make a pvc niddyy noddy or rig up any of several other ways to figure the yardage if you want.
The first thing to note is the seaming of the sweater you are looking at. There are two ways that sweaters are seamed, one is the material is knitted on a machine in large sheets, the pieces are cut/serged and serged together (like any other article of commercial made clothing). The other way is the sweater is knit on a machine, but with a precise pattern and one long piece of yarn is used (going back and forth up the sweater) and this long piece can be unraveled and wound into balls.
Here's what a serged seam (not useable for yarn harvesting) looks like:
And here's a chain stitched seam (what you want to look for):
The chain stitch is similar to the type of stitching that is done in the top of dog food bags that allows you (if you can find the tail) to rip the string right off the top of the bag and open it easily.
Here's a drawing of what to look for when getting ready to separate the pieces of your sweater. Find the side of the chain where the rounded part of the chain is closest to an edge. Slide a seam ripper under one of the chains and cut it (try not to nick the sweater yarn). You may have to loosen one or two more chains before you can start to pull the string from the other side of the seam and rip the whole seam out. Sometimes as your ripping it out, the chain might snag, just give a quick tug to both sides of the sweater and it should break free and you can continue ripping out the seam.
(more to come as I have time)