That is a tricky question. It depends on your water quality. More that the resin has to remove the quicker it will be depleted. On my awful water supply, the stuff would last a month. It can last longer for others.
If you don't replace the filters when they go bad, then you are almost using tap water. Is it better than distilled water? Most likely no due to the fact that distilled is processed where the tap wouldn't be...but it would also depend on how badly you let things get.
When it comes to the price of the replacement filters, it really isn't that out of control. I think the prices are reasonable. But then again, I'm also considering replacing my BRS unit with this one due to the size. I lost the unit as getting married made my bachelor kitchen needed room for two.
For me, I might get the 3 stage and add on the dual di to it. But very much in the thought phase with this, no matter how much I want to act.