Thank you คนถางทาง 

I had some dry kaffir lime zest, but never use any -- fresh is better ;-)

I had found that speed of fermenting depend on temperature (20-30C best), amount of sugar and ammonium sulphate (5-10%sugar, <1% ammonium sulphate) and the size of cut kaffir limes (smaller faster). I think types of microbes would play a big role there too. (I only tried 2 types: commercial 'actizyme'  and 'wild' microbes; both work OK.)

My current problem is: to measure "ripe-ness" of juice -- how do we tell when the juice is fermented enough to use. I will trying pH, hygrometer, smell, taste (yuck), electrical resistance, ... with this batch.

Once we have this pinned down, the we can talk "best fermenting".