if i were you I'd do complete enclosure disinfection now and place a frog on papertowels for now, keep it like that for a 2 weeks ( 14 days) ( changing and disinfecting everything every day) and then give him one dose of panacur, this way new worms won't get a change to lay eggs and he won't get a chance to get reinfected again. don't wait more then that, because worms will start laying eggs and you're screwed ( or your frog for that matter), it makes sense to do test about 2-3 weeks after last treatment, but when not many worms are present or they are not mature enough to lay eggs yet the test may be back negative. they are testing for their eggs not actual worms and if your frogs poops only in water chances are there will be no eggs ( or not enough) in the sample to show positive results.
believe me pinworms/roundworms can be a headache to get rid of.