This feature doesn't exist but you could easily write a user script that saves your file periodically. The follow code will save your file ever 5 seconds after your last key press.
var _ = require("contrib/underscore");
var debounce_saveMyFile = _.debounce(
window.addEventListener("editor_text_modified", debounce_saveMyFile, false);
When creating the Userscript make sure you set it's Trigger property to "On Startup". If you manually run it you need to uncomment that second to last line.