Usually lasts until I kick the container over yet again
Iv had it last for a year or so then it grew some sort of gunge on the surface so guess it was past it sell by date.
Have a look for Scale Clean or similar products used in the coffee trade its a lot cheaper and comes on 1kg bags and its citric acid
This site is a bit expensive but you can see what Im on about
https://www.thewatercoolerwarehouse.com/products/scale-kleen-1kg