The first RO system I ever had, I received no instructions with it, so never changed anything! Funnily it only lasted a year before I had to throw away the whole unit.
I learnt the hard way, never again.
I live in a very soft water area (yippeee!) but the water is very brown! so I now change my sediment filter about every 6-8 weeks, the Carbon ones about every 3-4 months.
Alex