How Fast Are Gravitational Waves?

Physicist Ethan Siegel answered a question I had never thought about: If a heavy mass suddenly materialized out of thin air a million kilometers away from the Earth, how long would it take for the Earth to feel its effects? In other words, how fast does gravitation act?

The answer in a nutshell: gravitational waves travel at the speed of light, so it would take around three seconds before we felt the impact of the new mass on the Earth.