gotta be > 800 ft, 'cause that's where my Gwiz kicks out the main
I'd say it depends on your recovery area, the wind and maybe the size of the rocket. DD is all about not chasing your rocket (and playing with explosives, rockets and electronics all at the same time).
For me, this is probably >2500 ft. or so. In one of our launch areas, it's required for >5000 ft to minimize recovery in a certain area.