I currently use something that looks like this:

My unfiltered tap water TDS is 210.
We use about 3000 litres per week (about 600 litres per day).
My current RO produces enough water - no problems - I leave it running pretty much 24/7 and waste water isn't a factor, but the RO filters were replaced in January and have now gone. I have changed the pre-filters every three months.
So my questions are:
1. Shouldn't my RO filters last longer? If so why do you think they've only lasted six months.
2. If a new RO/DI is needed, what should I get given the factors above?
Thanks in advance.