Lagrangian Orbit Question

[PlutonianEmpire]

Lets say there was a 2 Earth mass planet inserted into the Lagrangian L4 point of Earth's orbit. How it got there is not important. How long before the orbit of either planet becomes unstable?

[ajtribick]

Why not check out Gravity Simulator? It comes with a simulation of the solar system's planets, it shouldn't be too hard to figure out how to insert a planet into one of the Earth's Trojan points. I'm sure that if you are using an operating system that is not compatible with that particular piece of software it won't be too hard to find an alternative.