It also depends on what type of R/O you have.
I use a 1000l IBC tank at home to store the water in and my water pressure is high.
It takes over 24 hours to fill completely.
Waste to pure is approx 3:1.
The R/O is currently bringing my TDS reading down to 0.003 ppm.
But other R/O's will work much faster, the 4040 is the big one and will fill your 650l tank in a couple of hours with the R/O bringing the TDS down to about 0.001-3 ppm with a pure to waste ratio of 1:1
So it really depends on what R/O you have (plus your water pressure of course)
Regards central heating, how you open your stop cock doesn't affect the water pressure, only the rate of flow.
Your system will be fed by a header tank which will have it's own ball valve, so how far you open your stop cock shouldn't make one iota of difference.
Ian