What is the average life expectancy of a spider plant?

Spider plants can live many many years. Regular watering and lots of filtered light is important, of course, but it's a very forgiving plant if you mess up and forget every once in a while. It will rebound from nearly any mistreatment. I even accidentally froze mine to the window once.

Once it becomes root-bound, move it to a bigger pot with adequate drainage. This will help it stay big and healthy. Eventually it will make baby plants - if you propagate these you've got a practically infinite lifespan for this plant!