I'm not familiar with the exact brand you are using, but I continue the dosage until the ich is no longer visible. If you can see it, it obviously isn't gone yet, so don't stop. If the meds don't kill the fish, the disease will - take your choice. *shrugs*
Depending upon what or how bad a fish has something when I am treating them, depends on how often I do water changes. And yes, I do water changes during a treatment regimen, because six days (grabbing that number since it's what your med seems to say) is too long to not have a water change. A sick fish still poops, and so the water still gets dirty. Great, you saved your fish from dying of ich, instead it died of ammonia poisoning (not directed at you or anyone in particular, this is just a generic 'you'). You can't just focus on one thing and forget about the rest, they don't go away.
Being as when I treat my fish, they are in a quarantine or hospital tank which is not very large (1 - 2.5 gallons depending upon which tank they get stuck in) means that they generally get water changes every third day at most. But hey, these fish are sick, they generally got that way from dirty water, at least that's my general theory and thereby methodology in dealing with these things. So cleaner water = better fish? In severe cases of ich I've tried doing 100% water changes ever day over the course of treatment - the betta gets scooped up in a cup, the tank gets dumped and washed in HOT water, hot as you can get it, and then dechlorinated and salted water is added back, the fish is put back in, and then meds are readded. Eh, so far I haven't had enough ich cases to treat with differing water changing methods to come to any conclussion on which is better. I've saved fish with both. However in treating cases of finrot, the 100% water changes every day for a week straight (with salt in the water) have done wonders in clearing that up pronto. But that's not pertinent to this topic I imagine.
On irritating the fish or not: it's difficult to tell exactly what a fish may be feeling sometimes, we're only guessing. I have had several bettas that I have treated with quick cure though that after the first or second day will purposely swim to the drops as soon as they are added to the tank (since Quick cure takes a little bit of time to disperse out evenly into the water). These drops are obviously more concentrated then, logically if the fish did not like it they would swim away from it, the tanks I keep them in aren't so small that they can't move away from the drops. To me, this behavoir seems to indicate that there is something they like about the stuff - probably the itch/pain relief it causes by causing the parasites to go into 'shock', die and release their grip on the fish.