Hiya... I heard you have to replace filters annually anway to avoid bacteria build up... Not sure how true that is but makes sense to me.
The item I sent a link to is just a TDS meter that is inline. You connect it infront of your filter to measure incoming TDS and after your filter to measure outgoing. You can then check take readins each month and if your TDS leaving the filter starts to rise more than the usual daily fluctuations you find in the mains water then it is no longer softening the water as effectivley and you can then replace. It is the same logic as using the tables supplied with the filter and then comparing to water usage but I think it is a bit more accurate. RO has nothing to do with it, you can fit them to any type of water filtration.
Here is another link with a bit more info:
http://www.tdsmeter.com/products/dm1.html
I saw one on amazon a few months back. I'm getting one soon so I know when to replace my filter.