Depending how much water you produce, they will last you a couple of years before you need to change them.
Providing you back flush regularly, and change your pre filters often, they will last a long time.
After a while your TDS after the water has passed through the membranes (but prior to it passing through your blue, resin chamber) will slowly begin to rise.
This WILL happen eventually, regardless of how often you back flush and change your pre-filters, it is then up to you at which point you decide to replace the membranes.
I'm on my second set of membranes (they are the same as yours) and my TDS from then has now climbed to 0.007 ppm, they were reducing the water to 0.001 ppm when they were new.
For me, when the TDS gets to about 0.010ppm I will replace them, they are not particularly expensive.
It is the balancing act between the extra resin you are using against the replacement cost of the membranes.
Though I may well upgrade to a 4040 R/O as I am now needing to produce more water.
At what point you decide to replace the membranes is entirely down to you
Ian