The incubation period for Covid 19 is 7 to 14 days. What that means is that it could take up to 14 days for symptoms to appear. I believe that also means it could take a similar amount of time for you to test positive, from the date of exposure. The PCR test is better than nothing, but all it proves is that you are negative at that moment in time.. There are countless cases of people who test negative and then test positive a few days later.