It will be impossible to tell you accurately, it is not an issue of time. Source water is a huge part of the equation.
I once lived in a place where I had no RO unit, and just ran straight DI. That DI would last me about a year. But the TDS out of the tap was 8.
My current location my TDS out of the tap is over 500 sometimes, so my RO filter doesn't last as long, and neither does my DI.