It takes like 2 to 3 days maybe even a little longer
You don't mention the type of parasite or what was given and severity can matter so it's impossible to say. Different parasite's require different types of treatment's though in many cases a second course is given at the appropriate time or the cycle will start all over again. If the wrong treatment was given (often occurs when giving an OTC yourself along with some other scary things) then it may not be doing anything at all.
My puppy is having the same problem, I gave him worm meds at noon and now at this time is still doing the same thing.