Best results are at 20% mix.
Depending on how bad the conny is, I apply via a hand pressurised pump, no dwell time, simply start scrubbing - for lightly soiled jobs. Heavily soiled and ingrained jobs I apply through a backpack, we soak over the area with the mix, then 'dry' scrub ie.. Work the mix in with water fed pole brush, then turn on water and scrub clean.
For a conny that's pictured above in a really bad way would take around an hour and 1/4 for a complete clean top to bottom (finials, spars, panels, guttering, frames, windows and sills)
Darran