Shoot for around 18".
It varies. You would think it should be about 24-25" (which is a standard counter depth) but in reality many are closer to 27".
A table tennis table is 2 feet 6 inches (76cm) high. The net is 6 inches (15.25 cm) high.
A table tennis table is ... 9 feet (2.74m) long, 5 feet (1.525m) wide and 2 feet 6 inches (76cm) high ... and the net is 6 feet (1.83m) long and 6 inches (15.25 cm) high.
5 ft - 9ft is the size of an ITTF regulation table tennis table.