EE's autosave is not entirely clear in the Docs. Here are the facts:
Autosave (tested versions 2.5.5, 2.6.1, 2.7.2) is on by default. http://ellislab.com/forums/viewthread/222582/#1023950
You can 'disable' it by a hidden config var either in a bootstrap file, to your config or through the CP global vars. Set the value to 0. http://ellislab.com/expressionengine/user-guide/general/hidden_configuration_variables.html#autosave-interval-seconds
In my testing the default is set to 60 seconds. (I haven't searched through the source code, this is from using a stop-watch).
What's not clear in the docs is what triggers an autosave. The timer starts from the moment a value is added to an entry field (Publish/Edit screen). So editing an existing entry does nothing until something has been altered on the page.
To test: edit an existing entry, change the title, wait 60 seconds, autosave posts a message next to the 'submit' button. (in testing changing autosave-interval-seconds to 10, works as expected).
When amending an RTE or WYGWAM field autosave does not execute.
Any help confirming this, help from Ellislab or Pixel & Tonic would be appreciated.