You really need to measure what you're getting out at each stage with new media, then track how effective each is over time. You'll soon work out a pattern, and know when you should be looking to replace various media. The most common one to change is the fine particle filter. Change this out as soon as you see some colour to it. Mine's brown after just a couple of months. It's cheap and quick to change. Keep the first couple of stages fresh, and it will save you a lot on the expensive membrane. When you change the first stages, and you still see higher TDS coming out, it's time for new resin or membrane (you need to measure the output of each to know which).