It's hard to say from the test strips. Generally the higher the TDS, the more often you'll need to replace, but that's only a very rough generalization. What is your TDS?
You have a basically two options on how to replace filters. The first is you can replace them on a schedule. Replace the prefilters every 6 months, replace the RO membrane every 2 years or so, and replace the DI resin when the TDS goes above zero. This will basically ensure you always have clean water, but it can be pretty wasteful.
The other option is to test. Every filter in your RO unit does a very specific task, and you can tell when to replace them by testing.
The sediment filter's main job is to protect your carbon blocks from plugging before they lose the ability to process chlorine. To know when your sediment filter needs replacing, put a pressure gauge before the prefilters and put one after. When the pressure starts dropping more than 5 - 10 PSI, replace the sediment filter. If the pressure doesn't go back up after replacing the sediment filter, the carbon blocks may be plugged and may need to be replaced as well.
The carbon blocks are designed to remove chlorine and other VOCs from the water. These are important because chlorine is really damaging to your membrane. To know when carbon blocks need replacing, test the brine (the waste water) of your RO unit with a chlorine test strip. If there is any chlorine, the blocks need to be replaced.
The RO membrane does most of the filtering in your RO unit. It will also probably last the longest, especially if you keep up with the prefilter changes. To know when your RO membrane needs to be replaced, test the TDS before the membrane (but after the prefilters) and test your TDS after the membrane (but before the DI resin). Divide the "before membrane" by the "after membrane" number. This gives you a rough percentage of the solids that the membrane is letting through. I would replace the membrane when the number is between 0.05 and 0.10.
The DI resin is the easiest to know when to replace. Simply test your TDS after the DI resin. If it's greater than 0, replace the resin.
I very much prefer testing to blindly replacing the filters every 6 months. As an example, my prefilters were last replaced in may of 2017. I still have great pressure and no chlorine breakthrough. My water's pretty clean, so this won't be the case for everyone. But it's a good reason to consider replacing the filters only when the tests say to do so.
EDIT:
here's a source for the above info from one of our water filter vendors.
