Very far, it's huge.

To see the entire Earth as a globe, one needs to be far enough from the planet to capture it in a single frame. The best-known image taken from such a distance is the "Blue Marble" photo, taken by the crew of Apollo 17 in 1972 from about 29,000 kilometers (18,000 miles) away. This image shows Earth fully illuminated, a rare view because it requires just the right angle and distance from the sun.

Satellites positioned in geostationary orbit, about 36,000 kilometers (22,000 miles) above Earth, can show nearly a full hemisphere, but to see Earth as a fully illuminated globe, astronauts or satellites would need to be much farther, about 40,000 kilometers (25,000 miles) away or more, and the lighting has to be just right.

Blue Marble (1972)