It will of course depend on the size of vessel and the ammount in the resin bag.
A 25 ltr bag lasts me over three months if I use 200 ltrs of water a day. My TDS is arround 50ppm. I do use 2 vessels in tandem, when the TDS of one starts to rise.
I run the water through this one first, I carry on using the old resin untill the TDS reaches 45ppm, then I dump it.
MY First in line vessel is now reading 4 ppm. I don't want to clean windows with that. So it's tap to first vessel, out of first vessel at 4ppm and into the second vessel
out of the second vessel at zero ppm. Your saving on resin as long as the tds out of your first vessel is less than that of your tap water. Dai