With robbtoberfest's original 3 bottle system I could get water to about 135 F in 3 hours. If a larger outside bottle is used (a juice bottle), the temperature could reach about 150 F. If I used a 2 bottle system with a cut out half of a car window shade reflector, I could go as high as 196 F. Unfortunately, I think the water may have boiled and it melted and deformed the bottle.
I also have changed the outer bottle to type of PET with a wide mouth so that you don't have to cut it open and tape it after each use. You just screw the two halves together.
Although I would like to use just PET (like soda bottles) bottles due to their low weight and abundance, I now use HDPE (like the cloudy Nalgene bottles) for the inner bottle. (Lexan bottles may release estrogen like substance so I avoid those.) I also will investigate using a lighter reflector - mylar with a collapsible plastic frame.
A 3 bottle system may insulate the bottle better when the weather is cold. I will test this later.
SEE THE COMPLETED INSTRUCTABLE ON HOW TO BUILD A VERY EFFICIENT ONE BY GOING TO THIS INSTRUCTABLE LINK: www.instructables.com/id/Solar-Water-Heater-for-Backpacking-Using-Water-Bot/
Solar Water Heater for Backpacking