read the back of the package.
if these are nu well acid tabs from johnson screen the water will actually change color when the acid has acted to its fullest potential. i think that it goes from deep red to a light orange.
also it is important that after you place in the tablets that the well get good and stirred up. you could rawhide the well or you could pour several five gallon buckets of water down after the tabs.
we had one guy stick a garden hose down the well and continually circulate the water, i don't think that this will work though, not a good idea.