I'd say 50-100 landings.

I think it depends on the airport (runway length, one piece of pavement vs multiple for crosswinds, etc, towered vs non towered), weather, how oftenthe student is flying. Also, when I was a new CFI it took me longer to know when a student was ready and I tended error on the side of caution, and presumably I teach better now lol.

Interestingly, last summer I soloed my first and only glider student in 18 flights... So only 18 Landings. And he had perfected landings a bit sooner than I felt he was safe with the tow.