The fluval drop checker I have now says every 2 to 4 weeks for changing the liquid. Only been using it for one week so not sure what happens to the colour when it stops working. The drop checker has a round sticker (right side of box in link) which basically has ok, low and high. From reading "ok" (green) is 25 to 35ppm roughly. So not the greatest accuracy it seems. And of course I can't really tell the green shades either. But it will annoy me if the chart says one thing and the drop checker says another and I can't explain why or put it down to error. At the moment they disagree by ~15ppm.
http://www.amazon.com/Fluval-A7551-CO2-Indicator-Kit/dp/B0052M9886
From post #31 of link below. From reading below it does seem that at higher ph values the chart becomes invalid. I "think" I have read somewhere that this is the case with kh as well. At low ph and kh (all else being correct), the chart is more likely to be correct.
"KH kits measure HCO3, CO3, and other pH - resisting buffers. At a pH lower than 8.32, ~100% of the carbonate buffer system is HCO3 and little CO3 exists. So CO3 is eliminated, which is a good thing. This means at a pH higher then 8.32, the chart is invalid since our KH kits are affected by both CO3 and HCO3 - we only want HCO3. The same logic applies if we have a lot of other non-carbonate buffers, which influence KH and make it appear we have more HCO3 than actual."
The PH-KH-CO2 equation completely wrong? UPDATE on 22nd post - Page 3
I guess where I'm trying to get to is understanding what makes the chart work or not work (while leaving the chemistry to the experts lol). For example over summer here the tap ph and kh tends to be lower. Over winter it climbs so I could just see myself finally being happy and then get a tap ph of 8.4 which mucks up the chart (as a rough example of Murphy's Law in my tank).