I have to agree with NetMax (I almost called you Netflix, hehe) that it
really depends. For the average household aquarium without
extraordinarily expensive fish I find that about 3-4 weeks is generally
enough time for pathogens and parasites to present themselves. Of
course, if something does turn up you should treat your quarantined
fish and then repeat the 4 week quarantining period. Everyone's number
is different but I wouldn't do any less then 3 weeks for new arrivals.