If I just fill the bins with a good mixture of Carbon and Nitrogen and don't turn them, it takes about 12 to 18 months to break down. Depends on the temperatures and precipitation. Every time you turn it you can cut that roughly in 1/2. If you have a great mix of finely shredded material with the right amount of moisture and turn it daily, it can be fully broken down in as little as two weeks.
I can see some pros and cons to using the
pond liner. Pros are that it collects heat and holds moisture. Cons are that it blocks oxygen from getting in to the sides of the bin, and I wonder what substances could leech out of it and into the compost. I guess my opinion would be to ditch the liner. Oxygen is the key ingredient to high quality compost. Won't matter if you are turning it often, but I am lazy and prefer to wait for it while focusing my efforts on making more. Often I only let it break down 1/2 way and use as sheet mulch or hugelkulture material. I just like to get it hot once to pasturize out the seeds and pathogens. The bits of material I loose through the gaps I simply pile back on top.