I've still got half a sack left which i started using march 2006.
The water after ro is 7 after i've flushed it through.
I use to go through it a bit quicker, but i disconnect from the di when i flush the ro and then turn the valve to 'production' until it is reading 7. Then plug it back into di.
The other post about deteriation of resin caught my eye as well.