Depends on your initial tds.
If low membrane could last 4yrs or more if looked after, if high could last 1-2 yrs also you need to factor in the pre-filters changed roughly every 6 mths as you must have carbon pre-filter to safeguard the membrane against chlorine, also if water pressure is low then you may need a booster pump and there's the leccy to run it, and last but not least there is the cost of the water if you are on a meter as the rejection rate maybe 1 to 1 or as much as 4-1.
Hope all of this makes sense
Steve a