The minimum "safe" height for skydiving is 2000ft with most jumps between 2000-5000ft depending on the type of jump and experience of the skydiver.

In emergency situations it's possible to deploy at 200-300ft. and survive.

Basejumpers deploy way below 2000ft. with 200 or 300ft. not being uncommon but they have gear and techniques designed to open the chute very quickly.

Skydivers can fall at a rate of ~160 ft/sec. So in a worst case scenario you've got 2 seconds before impact.