RO System....when to change filters?

[Becca]

I purchased a 100GPD RO System from Ray about a year ago. It came with extra filters. But what I am wanting to know is when should I be changing these filters? I read that it is supposed to be once a year....but I probably only use about 20 or 30 gallons a week if that. So shouldn't the filters last me a lot longer then the recommended time? And how important is it that I sterilize the system like the instructions mention? Also I just purchased a TDS Meter....the same one that Gin and Ross bought which I read in another thread. If I go off the TDS Meter for when the filters need to be changed....what kind of reading will I be looking for?

TIA!