How long can I live outside of the US and keep my green card valid?

Generally speaking you will be safe to stay outside for less than 6 months. If you stay more than 6 months, it is recommended that you apply for re-entry permit, which will allow you to re-enter US after prolonged stay outside. However, re-entry permit itself is not a guarantee. You may still be asked to provide evidence that you have not abandoned your permanent residency if you stay outside for too long.