Honestly... spend enough time around you and everyone ends up corrupted.
Literal
Honestly... association [subject-が] long become-come-[when-と], every-last-one you [by-に] poisoned.
つきあいが長くなってくると 'as the association grows longer' — ~てくると marks a gradual temporal development. 毒される 'to be poisoned/corrupted' — passive of 毒す, used figuratively for bad influence. お前 'you' is a rough, familiar pronoun. The tone is exasperated but affectionate — complaining about a friend's bad influence.