A RO membrane can last up to 10 years if taken care of.
You change the sediment filter and carbon block every 6 months as they are what protects the more expensive RO membrane and you change the DI resin when it no longer reads 0 TDS. In your case the resin could easily go a year or more.
The $120 RODI-90-Refurb, refurbished unit is an excellent buy. If you don't already have a TDS meter its in the same flyer for $25.
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A sediment filter and carbon block are $23 for the pair every 6 months and the replacement DI cartridge is $18 maybe once a year. Cheaper than replacements for the tap water filter and you will get true 0 TDS water for a lot longer.