RO filter replacement schedule?

[tropterrarium]

Just got a 5 stage RO system from FirstRay with some flush rate reducer, to replace a DI set-up. Filter replacement schedule is:
Sediment filter 6 months/3000 gal
Carbon: 12 months/6000 gal
Membrane 18 months/9000 gal

I assume this is like oil-changes on a car: which ever comes first.

I am a rather low user, say 3-5 gals/day, so that is 1000-1500 gal per year, and don't want to put more in the landfill than necessary.

Are you sticking to the "official" replacement schedule or do you extend it? I am most inclined to stick with the official membrane schedule, but wonder whether sediment and carbon can be prolonged a bit more. Local water TDS is on average around 350.