How often do we need to change the RO filters and membrane?

Typical 3 Stage RO Unit.

* Change the prefilter and the postfilter every 6 - 12 months is recommended.

Recommendation is to change the 2 filters that are vertical every 6 - 12 months.
And we would recommend using ( 2 ) of the CTO 2 Carbon Block for both.
CTO carbon block.

And the recommended TFC membrane most people would use is the 50 gpd

But note it is not always necessary to change the horizonally mounted membrane, as the "membrane", can last 2 - 5 years in most applications.
How do we know when it's time to change the membrane?

To Check:
Membrane performance is best checked with a T.D.S. ( Total Dissolved Solids ) meter. A properly working membrane will show a 85% or better difference between the incoming source water and the finished product water. For example: Incoming TDS = 330 and After RO, TDS = 35. You can buy a TDS meter for around $ 25.00 to perform this test, OR you might be able to find a "nice" water treatment dealer who will test before and after samples for free, if brought to him.

And you would want to check the water as it's MADE, for accurate reading of the TDS.
You do this by turning OFF the valve on the storage tank, and opening RO faucet.
The water will slow to a pencil lead size stream, or fast "drip". That is the "product" water you want to test.

Also, a good idea to completely empty the RO storage tank once a year to prevent "TDS creep", which can occur is you rarely remove a large amount of water from the system at one time ( like a gallon or more ).
I've seen where customer never took more than one or two cups at a time, and over the course of a year, the TDS level in the storage tank increased to a high level, while the production from the membrane was still very good when tested with tank shut off.
And with tank Drained, is the right time to measure the air pressure in the storage tank, and add air if necessary to maintain 7 - 8 psi.
See this article - Tank Pressure Problem.

Various membrane sizes ( GPD ) rates are available.
The physical dimensions are standard and will work in any non-proprietary unit
And the most common residential sizes are these:
25 gpd
36 gpd
50 gpd

Or "WaterShield 50" ( Water Shield 50 ) uses S1764RS membrane
And like the Microline, uses special filters, S7028 and S7025.

Membrane Rinsing Directions

It says: Element must be rinsed under pressure before being soaked or stored in water.
The reason being they are shipped with a "preservative" ( = Sodium Metasulfite ), which if not fully rinsed, may have a "laxative" effect if consumed.
Discard product water 2 hours minimum ( some directions that come with new units have increased this to 8 hours, or multiple draining the holding tank before using the water ) before using for drinking water or food-related products.

What does this mean in layman’s terms?

****
Either DRAIN your storage tank, and then let it FILL UP and then DRAIN again... and then okay to use the RO water after the 2nd time it fills up,
OR
just leave the RO Faucet OPEN for 2 hours.... the flow will be very slow, but that is normal.
After 2 hours, shut off Faucet and let the storage tank fill up ( over night or several hours at least ).
Then it will be okay to use.

Sediment Pre-filter ( first filter your water enters ) can be Sediment Only = Sediment 5 micron

Second Pre-filter is also the Post Filter when only 3 "stages" are used ( diagram is showing 4 "stages" ) and it can be GAC or CTO carbon block.
And the CTO Carbon Block is recommended for this "stage", as the 2nd filter under most units. This should always be a Carbon Filter and in most cases the Pre-filter ( 1st filter ) should also be a Carbon Filter ( REQUIRED to be a Carbon filter, if your water source is chlorinated ).
see: CTO 5 Micron Carbon Block

The INLINE Filter ( final stage or Post Carbon Filter in diagram below ) used on some, not all units and is typically the 10" and has 1/4" threaded openings. see: Sealed Carbon 5 micron In Line

If yours does NOT have this "stage", then do not order one.
NOTE: Some new models use special In Line filters with built in "quick connect" for use with 1/4" or 3/8" tubing, and cost more because the fittings are build in. Also, the average RO system only has 3 stages ( pre-filter, membrane, and post filter ), and the "Inline" post filter is not used ( nor required ).

How do we know when it's time to change the filters?>

* You don't know unless you waited too long.....
Prefilter clogs, and water production drops.
And if in chlorinated water source, the membrane can be damaged, if prefilter not changed soon enough.

Typical RO and Filters Housings,
4 Stage,