Almost a year with mine and not a single hiccup. I had to flush the draw line once when I cleaned the cuvette but that is it. Never even calibrated it. Not once. Consistent numbers every time and I have used 4 different lot numbers for the reagents.
I do have a caveat though. The alkalinity reads 1 to 1.5dkh low and always has. I split the difference and say 1.25dkh. So my Trident reads 7dkh and I know my tank is around 8.25dkh. I don't give a rip if it is 8dkh, 8.2dkh, or 8.5dkh. I don't care if the Trident is only reading .5dkh off now (assuming the Trident is wrong and not my Hanna.) I can see the trend and can watch my tank. Things are happy and growing? Good enough. If it comes down to manually testing a tank, I don't do it. I know I won't. Over 17 years in, I know myself and my limits. I have way better things to do than test my tank once a week. So $600 for the convenience of testing daily and watching trends? Count me in.
I also set it up to only test half as often. Meaning reagents cost me less than $100 per year. Buy them on sale at Black Friday and the 6 month will last you a year. I actaully have an excess supply of Reagent A now. Like 4 bottles LOL