The ratio so far is 2 brine water to 1 pure water for me. It depends on what TDS level your tap water supply provides.
I am using garyj's 5 stage system and I have produced about 500 litres of pure water so far requiring a change of first stage, a mere $4.
I use the brine water to wash car, mopping and washing clothes, the brine water is actually better than tap water, just that it is not good enough for aquarium.
5 stage - 1st stage filter big particles - 2nd stage Carbon filter - 3rd stage filter finer particles - 4th stage membrane(This is the most expensive, so the first 3 stage will save the membrane), 5th state DI resin(also quite expensive).
3 stage - 1st stage filter big particles - 2nd stage Carbon filter - 3rd stage DI resin(expensive).