In the SpaceX January 29, 2020 Starlink mission, the fairing separation happened at the altitude of 104 km, speed 8636 km/h. SpaceX is trying to retrieve the fairing halves and reuse them.
104 km is a reasonable altitude, and I believe the fairing halves get even higher due to intertia, before starting to fall down. There's almost vacuum, and they surely develop very high speed when falling. I wonder how come the fairing halves do not heat up and get damaged on their way down? The 1st stage has entry burn to slow down, but the fairing halves have only parachutes to slow them down.