Typically, its a 3 week treatment. Treat for 3 consecutive days and then 4 days off, then the cycle starts again, 3 days on 4 days off for total of 21 days.
Now, the reason why you see the one treatment for seven days on the package is because they assume you are not performing daily water changes. Also, "supposedly" prazi does not denature or weaken each passing day. I prefer to do a good tank clean/water change and re-dose each day. Its been a long time since I've used Praziquantel for flukes. You must have sensitive fish, such as tetras or scaleless fish.
Eddie