Thats rather hard to say unless you tell us what your TDS reading is after it has gone through your R/O and prior to it going through your resin.
There is a formula to work out how long a given amount of resin will last, and it has been put on the furum before now.
Justin Ruggles was one who I am sure put it in a post on the forum many months ago, I know I have seen it on one of his posts on the other forum
So a little more info is needed before we can answer you!
And in the meantime perhaps someone who knows the formula will stick it into a reply for you!!
Ian