When looking for an RO, look for one that uses standard filters. Proprietary = way more money every time you have to replace the filters. The RO filter itself can last many years on softened water, but the pre and post filters should be replaced every six months.

If you have chlorine in your water, removing it is a good thing for your RO filter - it will last longer. Chlorine can attack the filter membrane.

Let me backpedal just a titch on that last comment (long day, little sleep). Your RO system should have a prefilter that is GAC - that prefilter will remove chlorine. If your chlorine levels are excessively high (if you live close to the water treatment plant, for instance) then you may have to replace the prefilter more often to provide protection for your membrane.