You really can never know that they are all gone, & probably they never are. If you are seeing lots of dead ones at first that is good. If the numbers start to taper off with the same protocols, that is even better.
I say keep up the fight more or less ongoing. Maybe a week of paracides for maintainence on and a week off - though I use a zapper for an hour every single day while at the computer, personally.
Really there are lots of unanswered Qs about parasite killing. Just keep at it. One egg can eventually turn into an army. Mho.