I will add to Raphael and ...

I will add to Raphael and Pablo's comments. Yes, it depends on several factors, such as, membrane types, Feed water quality, Pre-treatment measures to include micron size pre-filters employed, chemical pH adjustments, as well as, proprietary anti-scalants. Other factors include your recovery rates (ratio of Feed water/Permeate/Concentrate flows) and how often you perform cleanings, as the more you clean, the more the membranes degrade over time.

From my experience, I find that a "tech hack" is to utilize your "Clean-in-Place (CIP) System" to collect Permeate water and heat it up with a built-in heating element in the CIP tank to about 100 F and circulate the flow of water through the membranes in reverse at a conservative low flow for back-washing.  Likewise, if you find that the face of the 1st pass elements to be fouled, you can also replace (switch) these with elements on the last stage, but flip them over so that the fouled ends face the discharge end before you conduct a normal flow rinse cycle with heated water. In addition, you should make it a standard operating procedure (SOP) to flush your membranes with Permeate water before shutting down the plant for an extended period of time for maintenance, etc. Elements should only soak in Permeate water. 

Implementing all these procedures, can help extend the life of the membranes significantly. However, this also depends on how your R.O./ Desal system is designed to include the infrastructure for a CIP system with built-in heater element and valving to configure for regular cleanings and or flushing scenarios.  I have been able to extend membrane average service lives from 4-6 years to 9-11 years.  Your performance will be reflected in your "Normalized Data program software" which you can trend. Do not forget to conduct "Conductivity Profiles" for each element to ensure integrity for fouling or leaking "O" rings and connectors.  Hope this helps. Postulating is fine, but I find that experimenting is the best way to achieve results. Nothing ventured nothing gained. Should anyone wish for consult, you are more than welcome to contact me. Best regards.