It's surprising that there aren't any instructions for repeating.
I think in your case it's 6 days isn't it? I'd treat for 6 days, leave it a day or 2, do a 50-75% water change & then start the 6 day treatment over again but only adding around 50% as much medicine as there will already be some in the water.
BUT, that's just what I would do, & I'm by no means an expert.
If you haven't already I'd increase your tank temp to around 30 celsius as the Ich parasite simply can't survive in these temperatures, so as soon as it bursts out of the white cyst on the fish with it's thousands of parasite buddies then they should all be fried within a couple of hours, as well as being more susceptible to your medication.
Hope this helps