If you are referring to HIV, CDC does not have data on a specific percentage for this. It is extremely rare. More than 95 percent of persons develop antibodies within 3 months. A few cases of persons who have not developed antibodies until 6 months have been reported; this is most common in persons who have taken antiretrovirals for post-exposure prophylaxis after an occupational exposure to HIV (e.g., a needle stick) but seroconversion
in these circumstances is also extremely rare.
(I was asking what kinda people get into the 6months ranged zone, but what are they talking about??.)