Long enough for people to get to know and like them - the same as when you catch the same bus as a stranger every morning. Some posters, like some bus passengers, reach that point sooner than others.
Not a bit of it! It's only natural to form a liking for a poster by virtue of their interests, "manner", style etc and just as natural to form dislikes of their opposites. Several 'livewires" at my 91st virtual birthday bash 2 years ago seem to have fallen off their perches or been re-incarnated under a new name "Oopsadaisy, " Jalima", "PECS" Wherefore art thou?.