Though I'd hate to, because I wanna wear the install longer, I've always gone 6-8 weeks.
But it depends on the cornrow pattern. When I wear sew-ins, I use either one of the two stylists I know.
Weaveologist A uses a circular pattern that's tighter and takes longer to do. But the install can last 8+ weeks.
Weaveologist B uses a quick "prison rows" pattern-- six to eight cornrows going back. They're looser, but my hair grows out of them too fast. 4 weeks...6 weeks max.